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IT scandal exposes legal rule that made it easy for Post Office to prosecute the innocent

Lawyers call for changes to digital evidence rule that made it easier for the Post Office to ‘bamboozle courts’ and make subpostmasters pay a heavy price for its IT failings

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Two separate events in 1999 have become inextricably linked in the process of destroying the lives of hundreds of subpostmasters across the UK.

Although unrelated, together they were to have a devastating impact on law-abiding people, criminalising them due to a faulty IT system and a legal rule that denied them a fair trial.

In 1999, the government-owned Post Office began installing a new core accounting and retail system, known as Horizon, in thousands of Post Office branches across the country. The system, from Fujitsu, was seen as a revolution at the time, automating manual tasks such as bookkeeping.

At about the same time, a presumption was introduced into law on how courts should consider electronic evidence. The new rule introduced in 1999 followed a Law Commission recommendation for courts to presume that a computer system has operated correctly unless there is explicit evidence to the contrary. This legal presumption replaced a section of the Police and Criminal Evidence Act (PACE) 1984, which stated that computer evidence should be subject to proof that it was in fact operating properly.

The new rule made it easier for the Post Office, through its private prosecution powers, to convict subpostmasters for financial crimes when there were accounting shortfalls, based on data from the Horizon system. One barrister has said that, as a result, the courts were “bamboozled” by the Post Office. Innocent subpostmasters who were convicted paid a heavy price.

It wasn’t long after the introduction of Horizon that subpostmasters began to report unexplained accounting shortfalls. These business people, who own and run Post Office branches, were deemed responsible for the losses if they could not prove otherwise.

Because the 1999 legal rule meant that data from the Horizon system was presumed accurate, losses were therefore considered the fault of the subpostmaster, whether through theft or incompetence. Subpostmasters were forced to use their own cash to cover shortfalls or, in many cases, were prosecuted for theft and false accounting if they couldn’t afford, or refused, to make up the unexplained shortfalls.

The Post Office did not have to prove in court that the computer system was not at fault. Some subpostmasters served prison sentences, hundreds lost their livelihoods and there is at least one suicide linked to the scandal.

Over a 15-year period, more than 900 subpostmasters were prosecuted for financial crimes based on Horizon data. Many claimed that this data was wrong – and they were proved right years later in the High Court.

Although many subpostmasters suspected that the computer system was to blame, proving it would require expert IT or forensic accounting skills. In 2004, on the advice of a friend, former subpostmaster Alan Bates contacted Computer Weekly about the problems. In 2009, after contact was made by another former subpostmaster, Lee Castleton, Computer Weekly revealed the stories of seven subpostmasters who had suffered at the hands of Horizon (see timeline of Computer Weekly articles below).

But this was just the tip of the iceberg and Bates went on to lead more than 500 subpostmasters to victory in a multimillion-pound litigation in the High Court, which began in late 2018. The case proved that the Horizon system had bugs that could cause accounting shortfalls. In fact, High Court judge Peter Fraser described the Post Office’s stance that Horizon could not be the cause as “amounting to the 21st century equivalent of maintaining that the Earth is flat”.

Yet the legal rules meant the Post Office could prosecute without providing evidence that the computer system was not at fault.

Post Office knew Horizon had errors

It has since emerged that, during those prosecutions, the Post Office knew that Horizon had errors that could cause accounting shortfalls.

“The Horizon case proved that judges should agree to the disclosure of more technical evidence relating to errors and bugs,” said Stephen Mason, editor of the practitioner text for judges and lawyers, Electronic Evidence. “For some reason that I cannot understand, judges often refuse defence requests for relevant evidence. This happened in the case of Seema Misra. If the judges in Seema Misra’s case had ordered appropriate disclosure by the Post Office, the members of the jury might have reached a different conclusion about her guilt.”

Former subpostmistress Seema Misra was sent to prison while pregnant after being found guilty of theft in 2010. Her conviction was referred to the Court of Appeal by the Criminal Cases Review Commission (CCRC) and will be heard in March.

Unlike the government-owned Post Office, subpostmasters did not have the money to pay for lawyers or forensic accountants that could help them discover and prove the cause of accounting shortfalls.

Those that did get expert support had different experiences. For example, speaking to Computer Weekly in 2015, Andrew Clark, visiting professor in information security at Royal Holloway University of London, said that in 2005 he was called as a witness for the defence in a case brought by the Post Office against a subpostmaster accused of false accounting.

The digital forensics expert, who was hired by solicitors in relation to the case, told Computer Weekly that after he raised questions about the integrity of the Horizon system, the Post Office dropped the case.

The Horizon scandal is widely seen as the biggest miscarriage of justice in modern English history. It has also revealed, in stark terms, the inadequacy of the legal presumption that computers are always right.  

Paul Marshall, barrister at Cornerstone Barristers, said: “If the Post Office had been required to prove affirmatively that its Horizon system was working properly at the material time and it had given proper disclosure of Horizon error records, then it would not have been able to succeed in its prosecutions and its subpostmasters, with perhaps some small exceptions, would not have been convicted.”

In March last year, a few months after Judge Fraser’s ruling in the Bates and others versus Post Office case in the High Court in December 2019, the CCRC sent a total of 47 cases of potential miscarriages of justice to the Court of Appeal for review. This was the biggest group referral in history. Six former subpostmasters have already had their names cleared in Southwark Crown Court and the rest will appeal their convictions in the Court of Appeal in March.

And there could be many more. The CCRC said it has about 25 cases of subpostmaster prosecutions currently under review, while the Scottish CCRC recently took what it described as an “unusual step” of writing to 73 people with criminal convictions potentially linked to the Post Office’s Horizon errors. If the SCCRC concludes potential miscarriages of justice, it will refer them to the Scottish High Court, where appeals are heard.

But even this could just be scratching the surface of a wider problem of computer-generated miscarriages of justice. The 900-plus subpostmaster prosecutions based on Horizon data could potentially be just one group example. Computer systems are used in every corner of the economy and society and it is not infeasible that thousands of people have been prosecuted based on computer evidence that was wrong.  

Marshall said there was nothing to suggest the Post Office was unique in its ability to succeed in civil claims and criminal prosecutions on the basis of flawed computer evidence. “The weaknesses that the Post Office ruthlessly exploited to its own advantage were systemic, both as a matter of substantive law and of procedure (disclosure),” he said. “ It follows that it is very likely that there will be many others who have been convicted of offences or held liable in civil claims on the basis of unreliable computer evidence.”

Opportunity to change the rules

There is now an opportunity to change the rules, supported by a very powerful and public case study, and the government has at least revealed that it is listening. Last August, Marshall was invited by Alex Chalk MP, minister at the Ministry of Justice (MoJ), to submit a paper to the MoJ on suggestions for improving the existing approach to the proof in court proceedings of computer-derived evidence. The paper was submitted to the MoJ in November and is now published.

The paper recommends that when electronic evidence is used in legal proceedings, the party relying on the electronic evidence should automatically provide sufficient details of their systems to demonstrate that they are professionally managed.

Chalk has referred the paper and its recommendations to the Criminal Procedure Rule Committee chair and Computer Weekly understands that has triggered a process of consideration that will take months.

“Both the Civil Procedure Rule Committee and the Criminal Procedure Rule Committee must review the rules on the disclosure of computer evidence,” said Marshall. “The reason for this is that the existing rules on disclosure simply do not work, as the Post Office scandal painfully reveals.”

Marshall said the Horizon scandal was a major embarrassment both for the government and for the courts. “It is simply embarrassing for the judiciary, at all levels, that the courts have been bamboozled by the Post Office and its lawyers over such a long period,” he said. “Given the increasing pervasiveness of digital technology, there seems to be a pressing case for judicial education.” 

Stephen Mason, a barrister no longer in practice and one of the authors of the paper, has been researching and recommending change on the use of electronic evidence in court since 2004. He told Computer Weekly: “I have been trying to persuade the legal profession for over a decade to take the subject seriously. One can only hope that the Post Office Horizon scandal will change things.”

Mason said the scandal illustrates the importance of not taking the “reliability” of computer evidence on trust. “The purpose of a trial is to test the evidence, but clearly the evidence of the Horizon computer system was never properly tested until 2019,” he added.

“It is to be sincerely hoped that justice will now take centre place when dealing with evidence from computer systems and devices of all types.”

Timeline of the Post Office Horizon articles since Computer Weekly first reported on it in 2009

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