The Post Office Horizon computer system has come under fire again after being labelled “not fit for purpose”.

As part of a Post Office investigation into the system, independent forensic computer experts Second Sight identified issues with Horizon. In a report leaked to the BBC, it stated that the technology “was not fit for purpose in some branches” and that investigators had not looked for the root cause of errors, but rather accused subpostmasters of theft or false accounting.

The report also stated that not enough training or communication was provided to subpostmasters.

Responding to the report, the Post Office said: “Although we will not comment on any confidential documents, after two years of investigation it remains the case that there is no evidence of any systemic issues with the computer system which is used by more than 78,000 people and which successfully processed over six million transactions every day.”

More than 150 subpostmasters believe they were wrongly prosecuted or made to repay money due to errors created by the Horizon system.

One subpostmaster, Lee Castleton of Bridlington Post Office in East Yorkshire, was held responsible for £27,000 in losses in 2004, while Jo Hamilton of South Warnborough in Hampshire had to repay a £36,000 deficit found in her accounts.

South Tyneside subpostmaster Kevin Carter was suspended when £45,000 was found to be missing from the post office’s accounts. He believes the system logged some transactions twice, creating the deficit.

“We were made to feel like criminals when we hadn’t done anything wrong,” he said. “The Post Office is adamant that there’s nothing wrong with the system, but how can 150 people be wrong?”