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

By Tony Collins

Did the Post Office prosecute in the hundreds knowing most convictions were a foregone conclusion even with “not guilty” pleas?

If you own a franchise and your franchiser accuses you of stealing £50,000 or fiddling the books  on the basis of its faulty computer evidence,  you may need £46m to prove your innocence.

This was the cost to Post Office’s franchisees – sub-postmasters – of proving in the High Court that their franchiser’s main computer system, Horizon, was flawed and that its contracts were unreasonable and unenforceable.

But how many lone defendants facing a crown court hearing based on faulty computer evidence would have £46m at their disposal to prove the evidence against them was wrong?


As a group of 557 litigants, former sub-postmasters led by Alan Bates were able to establish in the High Court that the Post Office’s Horizon system was to blame for money that seemed to disappear from branch post office accounts. The Post Office had spent nearly 20 years denying Horizon was faulty. It prosecuted more than 900 people on the basis of Horizon data and took action in the civil courts to force payments of tens of thousands of pounds in many individual cases.

In seeking to prove Horizon was faulty, Bates’ legal team were opposed by four top lawyers representing the Post Office – David Cavender QC,  Lord Grabiner QC, Anthony de Garr Robinson QC and Helen Davies QC.  Unusually in any High Court case, the Post Office also had two firms of solicitors.

A single Post Office application to the court – to remove the judge Mr Justice Fraser – cost £500,000. The two sides’ costs in total were about £100m.

In fighting the Post Office’s claims, Bates needed a talented legal team (led by Patrick Green QC) and a former insider who worked at Fujitsu, Horizon’s supplier, and who was willing to give evidence against his former employer. Bates also needed the disclosure of thousands of Post Office documents. And the sub-postmasters were fortunate in having a judge, Mr Justice Fraser, who understood the minutiae of the software issues before him. He knew when Post Office’s evidence was incomplete and misleading.

How was £46m spent?

In the end, the victory cost the litigants about £46m which included £15m in legal fees. The rest went to Therium which had taken the risk of funding the sub-postmasters’ litigation and “after-the-event” insurers who had covered litigants in case they lost and had to pay the Post Office’s costs.

None of this money and expertise would have been available to numerous lone defendants whom the Post Office prosecuted.

Those imprisoned on the basis of Horizon evidence included 19 year-old Tracy Felstead and pregnant sub-postmistress Seema Misra.

The prosecution’s advantage

Bates’ legal team won a civil case. Prosecutors in the criminal courts go into hearings with the advantage of knowing that judges work on a “presumption” that computer evidence is reliable.

The onus is on the defence to identify the specific faults in the system that caused money to go missing in the case of a theft charge. This may be impossible without full disclosure.

A matter of ethics

Barrister Paul Marshall has made a study of the consequences of the Law Commission’s recommendations in the 1990s that computer evidence be presumed by the courts to be reliable – a recommendation that was accepted by Parliament in 1999 by repeal of s. 69(1) of the Police and Criminal Evidence Act 1984.  He said in his paper The harm that judges do – misunderstanding computer evidence: the Lee Castleton case” that if the prosecution fails to disclose to the defence faults in its computer systems there is little courts can do. He said,

“The courts are virtually helpless to impose effective sanctions for failure to disclose evidence if a party fails to comply with their disclosure obligations under the rules of court. It is to be noted that the origin of these rules is equitable in nature – that is to say they attach, historically, to matters of conscience. Compliance with disclosure obligations is ultimately a matter of ethics, as much as of rules.”

But what if ethics are in short supply?

If a computer is faulty, it’s obvious isn’t it?

Barrister Stephen Mason, in his book Electronic Evidence, has a chapter of evidence (6) that undermines the Law Commission’s rationale for its presumption.

The presumption in the law of England and Wales is,

“In the absence of evidence to the contrary, the courts will presume that mechanical instruments were in order at the material time’”.

The Law Commission, which advises Parliament on legal reform, has made it clear that “mechanical instruments” includes computers.

We’ve moved on from cash registers, haven’t we?

The presumption that computer evidence is reliable is based, in part, on a belief that computers are the same as stopwatches or older cash registers of the last century that either worked or didn’t.

But a large organisation such as the Post Office will have complex software-based systems whose flaws may remain hidden.

Mason’s book highlights many cases of computer failures caused by a range of problems such as input data flaws, unintended software interactions, operational errors, developmental issues and software errors, risks of errors when modifying software, security vulnerabilities and testing problems.

The cases he highlights show the vast differences between mechanical devices where faults may be obvious to the end-user and complex software-based systems where faults may go undiscovered.

Mason says:  “Faults in software and errors relating to the design of software
systems are exceedingly common. And while defects in hardware have been relatively rare, they are not unknown.”

He described the presumption that software code was reliable as a “delusion”. He said,

“Justice should not be based on concepts with no basis in logic or science.”

Marshall says the presumption is,

“as unsafe in practice as it is unjustified in principle”.


Many in the IT industry are astonished that the Post Office prosecuted more than 900 people. They find it especially surprising given that sub-postmasters are a group most likely to hand in or give to charity a £10 note they found in the street.

But the criminal justice system makes it too easy to prosecute on the basis of computer evidence from a faulty system. Indeed it encourages questionable prosecutions under the Law Commission’s defective “presumption” that computer evidence is reliable.

For the Post Office, it might have been easier in some cases to prosecute when Horizon showed money was missing than involve the system supplier Fujitsu in a thorough and costly investigation of a possible system fault..

In such cases judges and magistrates would not require full disclosure of errors and bugs. All that was needed was the verbal confirmation by an IT specialist that the system was working properly.

And judges or magistrates will, it appears, usually assume that large companies tell the truth, know of any bugs and design weaknesses in their systems, don’t cover up and don’t intentionally mislead,

It is hard to avoid the conclusion that, for most defendants accused on the basis of a large organisation’s computer evidence, convictions are a foregone conclusion.

How can a justice system that readily, uncritically and routinely admits flawed – indeed in the light of Mr Justice Fraser’s judgments, false and seriously misleading – evidence, command public confidence?”.

It is the right of every citizen if wrongly charged to be able to walk into a crown court, plead not guilty and stand a chance, however remote, of being acquitted.

Two scandals

There are two scandals: one involves the Post Office and its Horizon IT system, the other the Law Commission’s unwitting encouragement of wrongful prosecutions based on computer evidence that may be incomplete and misleading.

It is not enough for ministers to announce a lessons-learning review of the Post Office Horizon system and for the Law Commission to shrug its shoulders and say, as it does, that nothing is seriously awry.

The Post Office scandal cries out for a judge-led inquiry. And if the Law Commission does not update its presumption on the reliability of computer evidence it is, in essence, accepting that most children will understand computers better than the crown and magistrates courts. Children know  computers can go wrong in many different ways. Would that judges grasped this notion too, particularly before they advise juries and unjustly hand down life-changing sentences.

Tomorrow – How will the Horizon 900 prove their innocence  – a new stage in the Post Office IT scandal?

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.

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

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

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

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

  1. Thank you so much for posting this update.

    How extraordinary! I had no idea (but should have) that the Law is Dickensian regarding technology.
    I would have thought that, with the increasing use of computers in the courts, along with inevitable glitches, that a more philosophically sound approach would have obtained.
    It looks as if all of our bureaucracies need a really hefty push to change their routines, systems and procedures – even when lives and the well being of the innocent are at stake.
    The longer this scandal goes on, the more unacceptable facts are unearthed and the more we need Sir Peter Fraser to sort it all out and bring justice to the vicitms.


    • Thank you Zara. I too had no idea.
      I have a feeling the impressive Mr Justice Fraser, being the best person to chair any inquiry into the Post Office scandal, would be the last person ministers and civil servants would want for the task. Tony.


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