Denials that a Post Office computer system which suggested workers were committing fraud was plagued by bugs ‘amount to the 21st century equivalent of maintaining that the earth is flat’, a judge ruled today.

In the sixth judgment on the Bates v Post Office group litigation, Mr Justice Fraser ruled in favour of sub postmasters (SPMs), stating: ‘The approach by the Post Office to the evidence of someone such as Mr Latif [a former SPM] demonstrates a simple institutional obstinacy or refusal to consider any possible alternatives to their view of [the Horizon computer system], which was maintained regardless of the weight of factual evidence to the contrary.’

He continued: ‘This approach by the Post Office has amounted, in reality, to bare assertions and denials that ignore what has actually occurred, at least so far as the witnesses called before me in the Horizon Issues trial are concerned. It amounts to the 21st century equivalent of maintaining that the earth is flat.’

Fraser J’s judgment, which is almost 200,000 words long, concerns the Horizon system which was introduced across all Post Office branches in 2000. In group litigation, 550 former SPMs argued Horizon was unreliable, and caused unexplained shortfalls and discrepancies in their branch accounts. The Post Office denied this, asserting that the system was and is robust.

Fraser J found it was possible for bugs, errors or defects to have the potential to cause apparent discrepancies in branch accounts, and to undermine the reliability of Horizon accurately to process and record transactions as alleged by the claimants.

He said: ‘The Post Office’s approach to evidence, even despite their considerable resources which are being liberally deployed at considerable cost, amounts to attack and disparagement of the claimants individually and collectively.’

He added that personnel of Fujitsu, the IT giant which provided the Horizon system, had provided ‘wholly unsatisfactory evidence’ and there had been a ‘lack of accuracy on the part of Fujitsu witnesses in their evidence’. Fraser J has passed Fujitsu’s evidence to the director of public prosecutions due to his ‘very grave concerns’ about its veracity.

Last week it was announced that the case, which has dragged on for over three years, had been settled for £57.75m.

Post Office chairman Tim Parker said: ‘In reaching last week’s settlement with the claimants, we accepted our past shortcomings and I, both personally and on behalf of the Post Office, sincerely apologised to those affected when we got things wrong. We have given a commitment to learning lessons from these events, and today’s judgment underlines the need to do so.’

The Horizon judgment could influence 34 criminal cases against sub postmasters which are currently being reviewed by the Criminal Cases Review Commission. If these cases are overturned, claimants could bring charges of malicious prosecution against the Post Office.