The Post Office has ended its Horizon investigation and withheld the publication of its latest report on the accountancy program.

Part two of the Post Office’s report was due to be released to members of the working group on Wednesday 11 March.

A day before the release, the Post Office reported that it had completed its investigations into the 136 postmasters concerning the operation of the Horizon and that the report, revised by independent forensic computer experts Second Sight, would not be released.

Melanie Corfield, communications team at the Post Office, said: “Nothing has been found in any of the cases to suggest Horizon has not worked as it should.”

However, there are still 80 cases remaining in the mediation system. The Post Office has asked Second Sight to conclude its review of the cases so far, and report on those still remaining. Corfield added: “The report is for mediation purposes and will be shared with the mediation scheme applicants and their advisors; it is not for wider publication.”

The remaining 80 cases have now been put forward for mediation, however those which have previously been the subject of court rulings will be considered on an individual basis. Cornfield added: “We are looking to resolve the outstanding cases as quickly as possible and we remain prepared to discuss individual cases and our investigations with applicants, involving their MPs if they wish.”

The Post Office hopes that this decision will help to accelerate the conclusion of the scheme.

A spokesman for Justice for Subpostmasters’ Alliance (JFSA), an independent organisation created to raise awareness of issues over Horizon, said: “We were due to receive the report prior to the next mediation meeting on March 24. We have contacted the local minister and are currently waiting for a response. What is it that the Post Office doesn’t want us to see?”

Lee Castleton, who lost his Post Office in Bridlington, East Yorkshire, was held responsible for £27,000 in losses in 2004 but he is no longer part of the mediation scheme as the Post Office does not believe there is anything new in his case. “The Post Office is in denial, I think the Post Office has in fact stopped their investigation because something very serious about the Horizon system has been found,” he said. ”The report has cost a lot of money; we should all be able to view the report.”

Graham Ward, former owner of Rivenhall PO in Essex, who is due to have his mediation on March 24, has been battling against the system since 2008. “I think they have made a big mistake,” he said: “The Post Office went into this saying they want to be open, stopping the report a day before it is meant to be coming out is not being open. Does anyone have the strength to go against them? I imagine the Select Committee will want to know the outcome.”