A computer system used by British sub-postmasters was classed as “high risk” in an internal Post Office document, the High Court heard on Monday as a trial got under way to examine the reliability of the technology.

The state-owned Post Office is being sued by 550 current and former sub-postmasters who claim the Horizon IT system used in their branches caused accounting discrepancies and errors in their branch accounts for which they were wrongly held accountable. Some of the claimants were accused of theft, fraud and false accounting and a number received jail sentences.

The legal action has been split into three interlinked trials examining different issues. The first was heard last year and Mr Justice Fraser, the judge who is hearing all three interlinked trials, indicated that he is likely to hand down his ruling from the first case later this week.


On Monday, Justice Fraser began analysing evidence in the second trial, on the robustness of the Horizon computer system and whether it was likely to cause shortfalls in post office branch accounts.

Patrick Green QC, acting for the sub-postmasters in the second trial, told the High Court that independent IT experts had identified between 12 and 29 computer bugs in the Horizon system with “strong evidence of the bug causing a lasting discrepancy in branch accounts”.

He told the High Court that one internal Post Office document examining its technology strategy in January 2017 had classified the Horizon computer system as “high risk.”

In his written arguments, Mr Green claimed that the Post Office initially admitted that only three computer bugs that could affect branch accounts had been identified in the Horizon computer system. However, he said the Post Office’s own IT expert later acknowledged that 12 of the 29 bugs that independent experts had discovered could affect the system.

“The combination of Horizon’s admitted imperfections and the volume of many millions of transactions, is entirely consistent with the levels of errors reflected in the Claimants’ case,” wrote Mr Green.

He added that the Post Office had initially “conveyed the impression that only one branch was known to be affected” by one computer malfunction, but it emerged later that the bug had in fact affected 30 branches. This “reveals a frankly astonishing lack of knowledge of the effect of bugs within Horizon,” he claimed in his written submissions.

The Post Office contended on Monday that the Horizon system was “reliable and extremely unlikely to be the cause of any given shortfalls”. In its written arguments it said that the sub-postmasters “have yet to show any clear cut example of bugs in Horizon causing false shortfalls that affected any one of them, let alone bugs causing the shortfalls that they claim not to be responsible for”.

It alleged that the sub-postmasters’ suspicion of the Horizon computer system was “driven by natural human scepticism to technology”. “It is easy to blame the computer when something has gone wrong in a branch. It is common knowledge in this litigation that no IT system is perfect. This does not, however, mean that Horizon is likely to be the root cause of claimants’ shortfalls,” it said in its written arguments.

The Post Office also claimed that the 12 computer bugs identified by its own IT expert were “not material” in the context of the Post Office’s branch network, which has used Horizon for over 20 years in 11,500 branches to conduct 48bn transactions.

The trial, which is due to last for several weeks, continues.

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