Computer says ‘guilty’ – the scandals contained within the Post Office miscarriage of justice scandal

 4th June 2021

One of the successful appeal barristers in the Post Office miscarriage of justice scandal has given a powerful and important speech, which you should click on and read here.

Almost every paragraph contains devastating stuff – mistake and abuse, after mistake and abuse.

So immense a miscarriage of justice was the whole affair that it is difficult to get one’s mind around the scale of what went wrong.

I think there were three particular scandals that comprise the wider scandal – though this is not an exhaustive list.


One scandal is the extent of what went wrong and how long it took for anything to be put right – the number of people involved and affected, and the length of time it has taken for there to be any justice.

Here it should also be noted that had it not been for exemplary judging in the civil case by Mr Justice Fraser, there may still not be anything approximating any justice in this case.

A huge, horrible system failure of the English legal system.


A second scandal is just how many managers and lawyers in the Post Office knew that there were injustices – or did not care that there were injustices – but pressed on with the prosecutions and resisting the civil claims anyway.

Here the failure is not so much of a system but of individual professional decisions made by many who could and did know better.

The aggregate effect of all these bad decisions was immense – but each decision could and should have been different.

It is not good enough for those who made those bad decisions to hide behind any system failures – each should be held accountable for their individual decisions.


A third scandal is the most basic of all – and is more fundamental than the failure of the legal system and the failures of managers and lawyers.

This scandal is about human credulity.

This scandal is about how mere shortfalls on a computer system were capable of being sufficient evidence in-and-of-itself for postmasters and postmistresses to be criminalised.

Computer says: guilty.

Here the scandal is not about systems or decisions – but about the nature of evidence and proof itself.

A problem of general gullibility.

As the appeal barrister Paul Marshall says in his speech:

‘One of the features of these miscarriages of justice is that, in almost all cases, the only evidence against the defendant in question was a shortfall shown in the Horizon computer system.   If you remember only one thing from this talk, bear in mind that writing on a bit of paper in evidence is only marks on a piece of paper until first, someone explains what it means and, second, if it is a statement of fact, someone proves the truth of that fact.  

‘The simplest explanation for the Post Office scandal is that documents generated by the Horizon computer system were routinely treated by lawyers and judges as though statements of fact that were true, without bothering to consider how their truth should be established.  It was taken as given that what a computer record showed was correct. The shallowness of this approach is reprehensible.’


Even if the legal system had worked better, and even if Post Office managers and lawyers had made better decisions, there was always going to be a problem if such uncritical deference was given to computer records.

A computer should never be the one to, in effect, pronounce guilt.


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33 thoughts on “Computer says ‘guilty’ – the scandals contained within the Post Office miscarriage of justice scandal”

  1. I understand that following the failures by Roy Meadow the rules were changed so that guilt in criminal cases could not be proven on the uncorroborated evidence of an “expert”. Rather, confirmation is required.

    The computer can be thought of as an “expert” and treated in the same way.

  2. A fourth scandal is the complicity of the developers and testers of the Horizon system, in particular its accounting software. As someone who has developed and tested software, including basic accounting software, for over 50 years, I am incredulous that such ‘buggy’ software should be used in such circumstances. Were they unaware of the consequences of their failures? Did they continue to assure the Post Office managers that it couldn’t be Horizon that was wrong? This should also be investigated.

    1. This was investigated. They did not. POL knew of the problems.

      The software was being used for a purpose it wasn’t designed for. Whilst I initially had the same reaction than you, I don’t think it’s fair on reflection.

      I would add a different final point though – a big problem was people pleading guilty to crimes they didn’t commit. It’s a common occurance (more often when the facts are true but they don’t amount to an offence, generally because of lack of mens rea, or don’t fulfill an established test that it’s possible a non lawyer may not know, like “intention to permanently deprive” or something)

      I think it’s harder to know what to do about this.

  3. Thought provoking blog as always David, as was indeed your recent post which went into some detail about this and signposted the excellent Private Eye article which was a damming piece.
    I am shocked by this case and the extent to which people at the Post Office were prepared to act the way they did.
    It represents, for me, a complete systemic failure of the ability to speak truth to power.
    As you aver there must have been employees at the post office who did not do the right thing but protected their careers and ignored the plight of Post Masters and Mistresses, whilst knowing they were not guilty of what they were being charged with.
    As a result I think I would add a fourth scandal if I may, the fact that this behaviour cost lives and ruined lives of people who had done nothing wrong but use a computer system that was flawed.
    I hope that now, with the weight of evidence building about the reality of the situation, that prosecutions are actioned of those within the post office who were responsible (retired or not) for pursuing this when they knew it was not the fault of those they were charging. They should face the consequences of their actions.
    Which was not just sending innocent people to jail, into bankruptcy, into unemployment but also ultimately to suicide.

  4. Astonishing. Surely the most devastating blow for English justice in this generation. There are people, in the Post Office, Fujitsu, the civil service who ought to be put in prison.

  5. It seems to me that diminishing numbers of us are prepared to ask “why is that so?”.

    Are we all too much in a hurry to stop and think, even over matters that may have some impact on our own lives as well as other people’s?

    This could be the case. In a dual-career family, just coping with the BASICS outside working hours (relationship and child nurturing, house and personal lives admin and time off to be properly “human”) is hard enough.

    Thinking about more abstract concepts (like justice and the range of possible causes of a particular problem) may be just too hard to do for many over-stretched and under-supported people.

    1. The Post Office had an elaborate corporate governance structure, “The Three Lines of Defence”, that was put in place so well paid people would ask exactly those questions. This structure proved to be little more than window dressing. Too many people, in risk management and internal audit, took their attractive salaries and failed to ask the difficult questions that were their duty.

  6. Perhaps this cartoon should be shown to the courts in all such cases – although it’s specifically about election software the basic message is universal:

    “Don’t trust [voting] software, and don’t listen to anyone who says it’s safe … our entire field is bad at what we do, and if you rely on us, everyone will die”

    Which doesn’t mean that all software is always bad, but that any result from it should be understood as being contestable, subject to multiple sources of error, and needing proper confirmation, such as other sources of evidence, a well-tested track record of errors being detected and handled (we know the humans operating it will make mistakes, so if there’s no record of errors being corrected then it can only mean it’s rife with uncorrected errors AND that those responsible for it are being wilfully blind). In other words: everything that the Post Office cases didn’t feature.

    1. I think the xkcd is misleading to say “our entire field is bad at what we do”. The reality is that the field is extremely good at what they do, but attempt things vastly more complex than other fields, so lots goes wrong. This is an important distinction because the solution it prescribes is different. The xkcd suggests that the solution is to improve the execution of the task (maybe by getting better people or training them better), but the real solution is to change the task: stop trying to do things which are too complex (or stop expecting it to work).

  7. Also wonder how these errors (once discovered) were accounted for in the back office system. If they were corrected at some point (as you would expect to make the data consistent), how were the monies recovered from the ex-employees accounted for? Wonder what accounting firm would have signed off on this.

    1. This is an important point. Double entry book-keeping required a balancing entry for these fictitious movements. Suspense accounts are used for this purpose. Balancing items are posted there pending investigation and correction. The fate of the entries that Horizon bugs generated for suspense accounts is unclear, but it seems that they were never fully resolved, which would have required a detailed technical investigation that would have revealed the underlying problems. They were written off to the profit & loss account. That, I assume, was the route by which money extorted from subpostmasters boosted the Post Office’s profits.
      Second Sight, the investigating forensic accountants, believed that this was the role of the suspense accounts and wanted to follow this up but they were sacked, without any credible reason being offered. The real reason was clearly that they were uncovering unwelcome truths.

  8. Thank you for this.

    You make the key points very clearly. I would like to underline that, as someone who worked as a programmer back in the 70’s – when it was still just about possible to visualise what a computer did, step by step – and then had responsibility for managing ‘business processes’ at the other end of my working life, I have never been able to take seriously the argument/excuse “the computer says so!”. While I can understand, just, how non-technical people could be fooled into believing that it must be true, there were certainly analysts and experts who MUST have known better but found it more convenient to keep quiet.

    And that does not excuse the managers and hierarchy who were prepared to state – on oath words to the effect “no other complaints or anomalies have been discovered” when they either knew it was not true, or actively avoided discovering the truth. Heads should roll, pensions should be reviewed and – if necessary – legal action for failure to fulfil duties should result.

  9. The material in that lecture is shocking. You also hint at management failures, not only legal ones. I think lawyers in such circumstances can only do what they are instructed to do by managers. This is all private prosecution, remember. So there are a number of layers of dreadful management. First, the procurement of the system from Fujistu. It seems probable that there was awful incompetence here starting with the dumping of the project on the PO in the first place. How was it specified? What were/are the support arrangements? Who, actually, was in charge of the project from the PO point of view? What was the contract between Fujitsu and the PO? Did the PO know anything at all about what was wrong with the system and if it did, how was this fact promulgated in the organisation?

    Secondly, somewhere there was a security team that was responsible for ensuring that there was no fraud or theft from Post Offices. For them, it appears, the change from stopping people raiding the physical cash to a world in which sub-postmasters were offered up as culprits by a computer system must have been manna from heaven, especially since the culprits were self-employed, powerless and suspicious by background. The lack of the control over prosecutions by the CPS, now universal elsewhere, was missing here. What is deeply shocking, even after the thousandth hearing, is that the prosecuting authorities in the PO lied consistently to the sub-postmasters that they were ‘the only ones’, that there was no problem with the system. A supervising lawyer might have spotted this lie and stopped it dead, surely. As it was, successfully prosecuting sub-postmasters would have been regarded as a success – a triumph of technology and legal aggression over human frailty. Quite wrongly, of course, but government organisations are known for this sort of mess, especially without transparency, checks and balances.

    Somewhere, the management of Fujitsu is culpable, however distanced they are from the heat of the action by dint of contracts.

    Then there is the senior management of the PO, at all times since this started. First there was an inability to understand or question what was happening and re-align the organisation. Secondly, there was a massive, expensive, organisation-destroying cover-up. These people should be prosecuted for perverting the course of justice, fraud, whatever appropriate. Those responsible in government should be hounded from power, if still in it, or if luxuriating in Ermine, be stripped of it and thrown out on the street.

    And the victims should be properly compensated; paid far more than some pittance that the Treasury will allow to counterbalance the pain, poverty and punishment that they have suffered.

    The Post Office organisation should be subject to thorough reform. Root and branch. This is the work of years to come, but if this represents a regular protuberance of the state into the lives of the people, then the state is rotten.

  10. I have known several post office masters/mistresses over the years and always as a customer depositing very large cash sums from my business. All of them, despite my own difficult behaviour (I have Aspergers Syndrome) were impeccable in their dealings with me and my business.
    They were all caught up in this dreadful scandal and all were innocent beyond question.
    I have long admired the post office and it’s workers.
    The last bastion of inclusivity, workers of all stripes and persuasions, much the same as industrial workers pre 1979.
    With Royal Mail sold off, the demise of the written letters, the shutting down of Post Offices and leasing of shop space instead.
    These changes and a management of narrow minded, narrow skill set people the British public have lost an institution that was worth of the best of us.
    As for this dreadful and shocking episode, I would hope that those responsible will face jail time, but I suspect they will get a gong or knighthood instead.

  11. Thank you, David.

    I had been following the matter since 2019 but can only now grasp its enormity. Yes, I agree with Marshall that some people should face prosecution for obstruction of justice. And for fraud: they were attempting (and succeeding!) to extract money on fraudulent grounds. And what was the Post Office’s board (or whatever governing body is called) doing? How come suddenly all these Postmasters and Postmistresses (not, one suspects, a group of people especially given to fraud) were misbehaving? And what about the consequences: how could they expect to continue to recruit good people if the people they recruited were subjected to this approach?

    Those are issues of morality and business acumen.

    There’s also a legal point, not mentioned by Marshall. The prosecutions were prosecutions that the Post Office itself ran, using an in-house prosecution team which acted in defiance of justice, “affronting the conscience of the court”. Are the rules about such “in-house prosecution teams” satisfactory? Would transfer to the CPS have substantially diminished or even stopped this abuse? – I’d be interested in your views.

  12. “…had it not been for exemplary judging in the civil case by Mr Justice Fraser, there may still not be anything approximating any justice in this case.”

    Spot on. If you understand this, you understand the chilling implications of a government supporting national newspaper running the “Enemies of the People” headline.

  13. Just to add to my earlier comment. Paul Marshall’s speech is indeed essential reading. Moreover, in the absence of any more severe penalty, EVERY senior official in the Post Office in anyway implicated in this scandal should be required to listen to the Guardian podcast linked by Mr Marshall – and listen again and again until they are able to appreciate what Ms Skinner and her family went through. Then count themselves lucky that they will never have to endure something like that, and spend the rest of their lives seeking to make restitution.

  14. This story consists of a striking juxtaposition of those held accountable for matters they should not have been held accountable for and those not held accountable (so far) for matters which they surely should be. But a lack of accountability for the powerful and privileged and those doing their bidding seems very much a feature of modern Britain.

    Who was held accountable for the 2008 banking crash? The austerity that followed had severe consequences for some and was estimated to have caused an additional 1000 suicides in Britain.

    What about accountability for the Windrush scandal – the stories of some of the victims of that debacle are disturbingly similar to the stories of the Horizon victims (destruction of livelihoods, humiliation, loss of homes and possessions)

    Will there be accountability for Grenfell?

    The cynic in me wonders if public inquiries are designed to defer the question of accountability in the hope that by the time they conclude the public mood will have softened sufficiently that any follow up action can be avoided. That certainly seemed to be the fate of the Leveson Inquiry.

  15. Weren’t we assured recently by a tech writer – I remember you tweeting about it ironically – that by 2050 most legal judgments would be made by Artificial Intelligence? And I have seen similar claims about AI taking over routine medical diagnosis, decision-making in accountancy…

  16. Very disturbing reading but I’m certain that other than some handwringing and platitudes and perhaps even the odd resignation, absolutely zero real investigations will happen of the actual individuals in Post Office management who either by incompetence or through looking after themselves despite knowing something was wrong, prosecuted the innocent and obstructed those getting to the truth. The establishment will sweep it under the carpet,

  17. The actions of Post Office Ltd were the worst aspect of this by far. The Post Office has a long history of investigation and private prosecution. During the Horizon scandal the Post Office brought 918 private prosecutions against its own employees. The Post Office acted as a law unto itself with no independent oversight of its prosecutions. I sincerely hope that criminal prosecutions of those concerned do take place.

    Most of this happened while the Post Office was still a public authority. However, the privatised Royal Mail retains its investigation branch and legal department so similarly vindictive private prosecutions couuld be brought in future. The investigation branch should be disbanded and any suspicions of theft or fraud referred to the police instead.

  18. I had earlier heard the shocking Guardian podcasts. Paul Marshall’s speech explains that the rule of law did not operate in relation to the Post Office over two decades. The final paragraphs of his speech concerning what the government did or did not know add to the deeply disturbing questions flowing from all this. Why has the government refused a statutory inquiry which could compel attendance of witnesses and production of documents? And what are the implications of the recent collapse of the Hillsborough trial for getting to the truth of the multiple repeated failures of the state, its agency, and the judicial system in these cases?

  19. Plainly a disgusting display. Tim Slessor in his book ‘Lying in State’ describes what he calls Whitehall’s SODEM (Standard Offensive/Defensive/Evasive Measures). Seems to me the PO inherited some of this culture.

    We might also look at our legal system. Only a few days ago we had it made plain that to the lawyers it is perfectly acceptable to have policemen and lawyers lie to a public inquiry. It is merely an inquiry and there is no legal obligation to tell the truth. Indeed in an adversarial system there might well be an obligation to lie where you can get away with it. One would be failing in one’s duty to one’s client if one told the truth.

    As said the PO is now on the hook for millions in lawyers fees and compensation. Money that will come out the public purse. The PO got into this mess because it could lie. Maybe we might look to its internal security people brought up in the quill pen era of fraud investigation and perhaps facing redundancy. Rather too keen not to look deeply into a new fangled computer system. Then the computer people may have had incentives to defend their baby for fear of the sack. Then the lawyers taking on case after case when all in the trade must have smelt a rat. Why did it take so long before a judge finally twigged something was not right, surely his predecessors were not thick or careless.

    At the root is the fact that people can and do lie. In a corporate context the cure is an absolute obligation to reveal the truth – the documents in full and in a timely manner – whether you be mighty or lowly. Thirty years in solitary for any coverup or concealment or falsification.

  20. Another worrying issue raised is that many in senior management positions were “in the know” but raised no finger. Some may well have been just too busy as Linda suggests, but for others, this campaign will have impacted regularly on their jobs. Did they all fail to point out the errors? If some did, then we want to know who over-ruled them. If none did, we are no better off: was there a culture of groupthink, or did they all fall into the 1/3 of compliant individuals in Asch’s conformity experiments?
    Grant took several of his old pals on campaign, so that most nights they could chat around his campfire. They will have taken the mince out of his less practical notions and helped keep his feet on the ground. I sometimes wonder if that is what medieval fools were for. Instead, our modern Post Office management has connived to show themselves as fools and worse- venal liars, within a compliant establishment.
    David Dimbleby says The Blunders of Governments should be every minister’s bedtime reading, but digesting it should be a requirement for advancement in any public service department. And then annual re-reading or interview in annual mincing meetings by review panels. If you have the moral courage and strength to admit, accept and learn from your mistakes, then you can do the job and earn your salary.

  21. Is this a warning to us all? As a society we are increasingly reliant on computers and their programmes to manage our daily lives. Are we to blindly accept their infallibility? If so it’s just another step towards the dystopian society we have been warned about…

    1. Yes Nigel, you are right.

      I spent my working life in IT and watched while systems became ever more more complex and budgets ever tighter. Add outsourcing to the mix and you sometimes don’t even know who is responsible to investigate never mind resolve a problem.

      There are so many examples of computer fallibility. The judiciary really need to wake up – read the news, follow the consumer reports and carefully check their own statements & invoices.

      A few illustrations.

      The problems with the Npower billing system were well documented and eventually contributed to the demise of the business.

      My current broadband supplier replaced its billing system in 2018 and has been plagued with billing errors ever since.

      I have been charged for the same energy twice when switching suppliers.

      I have an account with a challenger bank where items from the statement are mis-described, missing altogether and where the statement does not even add up (by contrast my 90 year old father, a one time accountant, reckons he never spotted an error on his bank statements in the pre-computer era).

      The BBC ran a story about a young man who had his insurance premium increased when the monitoring device fitted to his car reported a speed of 105mph; he was able to produce cctv footage showing the car was stationary at the time.

      Another BBC story concerned a man being prosecuted for fraud after he made an insurance claim using photos of the stolen jewellery retrieved from an obsolete Samsung camera with dates on the photo showing they were taken after the date of the reported theft. Full marks to Samsung for disclosing than an obscure bug in the camera firmware cause the dates to be over-written in some circumstances.

  22. Isn’t it a conspiracy when so many people are involved, carry out the same activities and must have been talking and discussing what they were doing with each other, they certainly weren’t working in isolation?

  23. Private Eye and Computer Week come out of this with great credit. They exposed the weaknesses of the system and the vindictive blindness of senior management for years. They were prophets crying in the wilderness. Unfortunately, C of E clergywoman and CEO had no Christian charity to conceive she and the computer system might be wrong. A lesson to corporates: if PE is on your case you need to investigate internally. For Paula Bennells, the sackcloth and ashes look suits you and beg for forgiveness for the rest of life.

  24. As a software developer, the one thing that really staggered me about this case was that the Horizon accounting system was showing up discrepancies as soon as it was introduced, and yet the Post Office immediately presumed the brand new system was correct and their long-standing employees were lying.

    I would never assume that a complex system would work correctly as soon as it goes live, and would be highly receptive to any early problems reported by users. It doesn’t matter how well tested something is, large-scale use ‘for real’ will expose new issues you would never have imagined in advance.

    Once a system has been tried and tested for some years and adapted in response to issues, only then can it be considered relatively reliable, and likely – not certain, but likely – to be correct. But a brand new system should always be considered the prime suspect and the first place to look to explain any discrepancies.

    To presume that their new system is perfectly reliable, and that any discrepancies must represent fraud by the user, is a frankly breathtaking arrogance. May this painful example stand as a reference for anyone who in future finds themselves on the wrong side of a computer system.

    1. And even with established and generally reliable systems, obscure bugs, by their very nature, can elude detection for a long time. Where a dispute arises, and the matter is serious, I would expect technical staff to be analysing logs for each stage of the transaction’s journey and if that is not possible, or too expensive, the member of staff has to be given the benefit of the doubt. If an organisation chooses not to build or retain comprehensive and accessible audit trails then in the event of large losses it is the director’s who need to be held accountable by the shareholders.

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