Lords debate amendment to law on use of computer evidence in light of Post Office scandal

Peers will today debate a change to the law on the use computer evidence, which was used by the Post Office to wrongly prosecute subpostmasters without reliable evidence.

The House of Lords will today see a debate on what is known as the Mr Bates amendment, named after the campaigning subpostmaster Alan Bates.

The amendment to the Data and Digital Information Protection Bill, laid by crossbench peer Beeban Kidron, has cross-party support, with peer James Arbuthnot, who has been campaigning for the wrongly prosecuted subpostmasters since 2009, providing his backing.

In 1999, a presumption that a computer system has operated correctly unless there is explicit evidence to the contrary was introduced into law on how courts should consider electronic evidence. This was the same year the Post Office introduced the Horizon software, from Fujitsu, to automate accounting in thousands of branches.

The 1999 legal rule replaced a section of the PACE Act 1984, which stated that those introducing computer evidence should be able to prove the computer sytsem was operating properly. The amendment 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.

This led to about 900 subpostmasters being prosecuted and convicted of financial crimes after they experienced unexplained account shortfalls. Since 2021, about 100 have had these wrongful convictions overturned and hundreds more are set to follow after the government announced a law to overturn the remainder en masse.

Arbuthnot told Computer Weekly that Fujitsu’s access to subpostmaster accounts, combined with the complexity of the Horizon system, made the computer evidence used in trials unreliable.

“With programs containing millions of lines of code, nobody can understand the unintended consequences, which makes computer programs inherently unreliable even though they may appear to be working properly.  For there to be a presumption that they are working properly is wrong in principle,” he said.

“Maybe our amendment has flaws, but so too does the present position in law. Our amendment is a good start on hammering out the solution we need to reach,” he added.

With programs containing millions of lines of code, nobody can understand the unintended consequences, which makes computer programs inherently unreliable even though they may appear to be working properly. For there to be a presumption that they are working properly is wrong in principle

Paul Marshall, a barrister who represented the subpostmasters who successfully overturned wrongful convictions by the Post Office, said one of the factors that caused the Post Office Horizon scandal is the fact lawyers and judges failed to understand that computers “have a propensity to fail”.

“There is no basis in reality that computers are reliable all the time,” added Marshall.

As Computer Weekly revealed in 2021, following a response to a freedom of information request made by computer scientist Steven Murdoch at University College London, the Post Office wrote to the Law Commission in 1995 when the law change was being proposed, and said the rule at the time – that computer evidence should be subject to proof that it was in fact operating properly – was “somewhat onerous” when prosecuting people charged with crimes, such as the subpostmasters who run and own its branches.

Marshall added: “The change in the law at the turn of the century coincides exactly with the Post Office Horizon system’s introduction and the dreadful harm that it inflicted on hundreds of innocent people.”

He said the law needs to be changed to avoid a repeat of the Post Office scandal, which is seen as one of the biggest miscarriages of justice in British history.

The Post Office scandal was first exposed by Computer Weekly in 2009, revealing the stories of seven subpostmasters and the problems they suffered due to the accounting software (see timeline of Computer Weekly articles about the scandal below).

• Also read: What you need to know about the Horizon scandal •

• Also watch: ITV’s documentary – Mr Bates vs The Post Office: The real story •