Courts too ready to accept flawed computer evidence – which puts countless innocents in prison, say legal and IT experts.

By Tony Collins

A major defect in the criminal justice system encourages courts to jail people or inflict life-changing financial harm on the basis of faulty computer evidence, say barristers and senior IT academics.

The defect means crown and magistrates’ courts will accept computer evidence from big organisations as reliable even when it is flawed, incomplete or misleading. The organisation can follow a successful conviction with a civil action that forces payment of large sums.  Wrongly convicted people can then be forced into paying the organisation their life savings or the proceeds from the sale of their homes.

The Law Commission, which advises Parliament on law reform, has known of the flaw in the criminal justice system for more than 20 years but has not heeded warnings that it could lead to countless miscarriages of justice.

Post Office IT scandal

Now the warnings have turned to reality. In what is being described as the widest miscarriage of justice in British legal history, the Post Office prosecuted more than 900 people using computer evidence from its flawed Horizon system.

Based on Horizon evidence, the Post Office also pursued countless people in the civil courts, making some of them bankrupt and forcing some to pay tens of thousands of pounds for money said by Horizon to be missing but which may have been transferred by the system into a hidden central Post Office account.

Despite the Post Office IT scandal, the Commission has still not announced any plans to change a legal “presumption” that leads to courts’ treating the prosecution’s computer evidence as reliable.

The Government is taking a relaxed view of the Law Commission and Post Office scandals. It has announced a lessons-learning review of the Post Office Horizon system but no investigation of the Law Commission scandal.


Barristers and IT specialists say the Commission’s presumption is extraordinarily naïve and undermines confidence in the UK criminal justice system.

Barrister Paul Marshall has made a study of the subject. He has written a paper entitled The harm judges do – misunderstanding computer evidence: the Lee Castleton case”. He quotes one judge as expressing a “frankly bizarre and atypically silly” view that,

“… one needs no expertise in electronics to be able to know whether a computer is working properly.”

Full disclosure is rare

Under current legal advice to courts, the onus is on the defence to  prove flaws in the prosecution’s computer evidence but big organisations are unlikely to make a full disclosure of errors, bugs and weaknesses in their back-office systems. They will regard such information as commercially confidential. Full disclosure is rare.

In the Post Office’s case, numerous flaws in its Horizon system were covered up for nearly 20 years during which time it prosecuted at least 900 people on the basis that the system was robust.

Nobody has had their criminal convictions quashed or their life savings and  proceeds from the sale of their house returned.

Marshall describes the Post Office scandal in his paper as “mendacity on an epic scale”.

Frankly bizarre

Barrister Stephen Mason is a long-time critic of the Law Commission’s stance on computer evidence. He says said in his book  “Electronic Evidence” that the courts treat computers as machines.

“It cannot be right to presume that a machine, in particular a computer, computer-like device or network, was ‘in order’ – whatever that means – or ‘reliable’ at the material time.”

Mason has obtained a rare transcript of a criminal court hearing. It  shows how a crown court judge accepted the Post Office’s computer evidence on the basis of an IT-literate person from the company testifying that the computer was working properly at the material time. The testimony included a claim that it would have been obvious to the end-user if the computer was not working properly.

The transcript also shows that the judge appeared to be sceptical of claims by the accused, a pregnant sub-postmistress Seema Misra, that the computer was faulty. She denied stealing money that Horizon evidence suggested was missing.

Clear injustices 

Misra’s lawyer asked the courts three times for her criminal trial to be stayed as an “abuse of process” because of inadequate disclosure of the computer data against her. Each of her applications was rejected by different judges.

Instead, the court accepted the prosecution’s claims that Horizon was reliable. Misra fainted when she was jailed for 15 months. She was eight weeks pregnant with her second child.

The judge had told the jury that there was no evidence money was stolen. But he left the jury in no doubt about the reliability of Horizon. He told them,

“There is ample evidence before you to establish that Horizon is a tried and tested system in use at thousands of post offices for several years, fundamentally robust and reliable.”

Also incorrect was a statement to the jury by the Post Office’s prosecuting counsel.  Referring to Horizon, he said,

“… it has got to be a pretty robust system and you will hear some evidence from an expert in the field [Fujitsu] as to the quality of the system … the Crown say it is a robust system and that if there really was a computer problem the defendant would have been aware of it.

“That is the whole point because when you use a computer system you realise there is something wrong if not from the screen itself but from the printouts you are getting when you are doing the stock take.”

That the prosecution did not tell the judge and jury of numerous bugs in the Horizon system or that figures on branch accounts could be altered remotely without the sub-postmaster’s knowledge has led to no admonishment of the Post Office by any government ministers.

Pay Post Office £321,000 says court

In another clear miscarriage of justice, the Post Office won a civil case to make sub-postmaster Lee Castleton pay £26,000 that was shown on Horizon as missing. Castleton appealed but the appeal judge rejected any suggestion Horizon was not working properly.

He ordered Castleton to pay costs which the Post Office put at £321,000. Nobody has ever understood since how the judge allowed the Post Office to claim £321,000 in costs over a disputed sum of £26,000. Castleton was forced into bankruptcy and, to this day, the Post Office has never repaid the money he lost through the courts’ acceptance that the system was working properly.

Marshall says in his paper that the judge in Castleton’s case was satisfied with the testimony of a witness from Horizon’s supplier Fujitsu whose evidence “amounted to saying she herself could not see anything wrong with the system”.


Mason in his book explains how Toyota intentionally covered up information and misled the public about the safety issues with some cars that had computer-controlled accelerator pedals.  Toyota at first denied that its software was to blame for a spate of unintended accelerations, some of which caused fatalities, but after a dispute lasting several years Toyota agreed a settlement worth nearly £1bn.

The case showed how a big organisation may not fully understand its own software and have little idea of its coding flaws until a serious incident or spate of incidents occur. It is usual after such incidents, as in he cases of Toyota and the Post Office, for the organisation in question to deny a first that its software has been the cause of serious incidents.


But UK judges take little or no account of corporate cover-ups. Marshall quotes one judge, Justice Lloyd as saying,

“Where a lengthy computer printout contains no internal evidence of malfunction, and is retained, e.g. by a bank or a stockbroker as part of its records, it may be legitimate to infer that the computer which made the record was functioning correctly.”

Another similar comment was made by Lord Griffiths,

“‘…  in the vast majority of cases it will be possible to discharge the burden (of showing the computer was working properly) by calling a witness who is familiar with the operation of the computer in the sense of knowing what the computer is required to do and who can say that it is doing it properly.”

Mason says,

“The legal presumption that computers are reliable is ridiculous.”

Law Commission’s view

The Law Commission’s view is that the “presumption” does not place too great a burden on the defence. But the Commission’s critics say the numerous miscarriages of justice in the Post Office Horizon IT scandal are proof that it all but impossible for defendants to prove their innocence when they are against a big organisation’s computer evidence.

The Commission is also likely to deny that it has, with indifference, carried on with the “presumption” about the reliability of computer evidence. This is not how the Commission works. It conducts projects into areas of the law and produces reports, which the government is free to accept or reject.

The “presumption” is 23 years old and the Commission does not keep areas of law under continuous review. The Commission can be asked to look into any area of law again in a new project but cannot launch one unless it and the government agree on its need.


The criminal justice system ought to be reining in corporate mendacity not encouraging it. At least 900 unsafe prospections and judges and juries sending innocent people to prison would not have happened if courts had been more open to the defence’s challenge of computer evidence. That judges were too ready to accept computer evidence as reliable is obviously wrong. But the Law Commission seems content to be complicit in the governmental and civil service  pretence that there’s nothing seriously awry. How can the Law Commission go another week without starting work on reforming the advice to judges?

Every week that the legal advice to judges remains unchanged is another week that the Law Commission’s “presumption” on the reliability of computer evidence makes a mockery of the criminal justice system.

Tomorrow – If faulty computer evidence is used against you in court, you may need £46m to prove your innocence.

Donations for a QC’s complaint to the Parliamentary Ombudsman of maladministration by the Department for BEIS over its failure to stop the Post Office making accusations in the criminal and civil courts on the basis of its flawed Horizon system.

Electronic Evidence – Stephen Mason and Daniel Seng  (NB chapter six)

Presumption concerning the dependability of computer evidence – Ladkin, Littlewood, Thimbleby and Thomas

The harm that judges do – misunderstanding computer evidence: Mr Castleton’s story – Paul Marshall

Seema Misra criminal trial transcript

Post Office cover-up – they wanted it all to go away – Nick Wallis


4 responses to “Courts too ready to accept flawed computer evidence – which puts countless innocents in prison, say legal and IT experts.

  1. Thank you so much, Tony.

    As they say, the Law is an ass, and you have given us examples whereby the judges can’t help but evidence that observation.

    It doesn’t help that most in the judiciary are educated in the arts rather than the sciences – still a great deal of 18th-century snobbery going on there I think. Having a first-class Hons in Greek doesn’t confer an understanding of technology and science.

    In addition, it seems to be part of human nature to be dazzled by, and therefore to believe, those who are in a more powerful position irrespective of their other qualities. The courts are no different. They invariably defer to ‘experts’ whilst the firsthand testimony of lesser mortals tends to be dismissed.

    I am thinking of the awful case of Sally Clarke, convicted of murdering her children on the bonkers evidence of the flawed expert Professor Roy Meadows and his fictional figures. Clarke’s defence didn’t challenge this evidential nonsense at the time. We later discovered that being a court expert witness was an easy and lucrative occupation. There followed similar cases of child murder cases being overturned for similar reasons.

    Forensic evidence presented in court can be flawed – even fingerprints. But, it seems even defence lawyers don’t always have the scientific training, or even street-level instincts, to understand enough to appreciate what the limitations are of the techniques used and therefore challenge them.

    Brian Kelly, ex-police officer, was the first person in Scotland to be successfully prosecuted on the basis of DNA evidence and was jailed for six years in 1989.
    He was an innocent man – even the victim did not identify him as her assailant even though she knew him. Nevertheless, the court believed the flawed DNA evidence (cross-contamination), presented to the jury as fact, rather than the victim’s testimony.

    What was so impressive about Sir Peter Fraser was that not only was he an exemplary judge but, had an enviable understanding of technology. I still harbour the wish that he could lead a full judicial inquiry into the Post Office scandal.

    Wishing you, and the victims of justice, well..


    • Thank you Zara for details of those miscarriages of justice and for pointing out that Sir Peter Fraser, the judge in the Post Office litigation, has an exceptional understanding of technology. He also has an ability to cut through corporate mendacity to expose the truth or at least identify when the truth is unlikely to be known. Yes, he would be perfect for a judge-led inquiry into the Post Office scandal. Tony

      Liked by 1 person

  2. Another great article Tony highlighting the absurdity of the law as it stands in relation to software. Post Office Ltd and BEIS are attempting to draw a line under this scandal, hoping that it will go away primarily by saying that they have learned from their mistakes and that the current version of Horizon is reliable. It may very well be more reliable than the past versions but the presumption has to be that it may still contain unknown errors that can cause losses to be accrued to subpostmasters accounts. POL have clearly not learned that the only way to prove that the computer is not at fault is to prove without question how the loss was incurred. Proof of user and customer interaction with the system is required to do that and that means full CCTV coverage of every transaction just as you will find in every major organisation that deals with many financial transactions such as supermarkets, casinos and banks. POL can either install the CCTV at every branch or accept the risk for all losses. They can never rely on their software in court again. Cheers, Tim


    • Thank you Tim. Absurd is the word. Your comment is a reminder that the prosecution can make an accusation of theft without showing at a system level how it occurred. It need only testify that its IT systems were working correctly – and judges are content to refuse the defence’s pleadings for full disclosure of internal reports on bugs. The Post Office IT scandal highlights not only numerous miscarriages of justice but an unwillingness in government and the Law Commission to do anything about it.


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