Over 100 former sub-postmasters have claimed for some years to have experienced problems with the Horizon accounting system, with some facing court proceedings over alleged instances of theft, fraud and false accounting since the introduction of an updated online version. They have claimed that the problems were down to computer faults within the new software.

In 2012, the Post Office asked Second Sight Support Services to carry out an independent review of Horizon. The firm’s interim report found no evidence of ‘system-wide issues’, but the Post Office went on to set up a mediation scheme for postmasters to raise specific concerns.

Second Sight was then asked to produce a further report examining some 150 individual complaints, which was delivered this week. The Post Office has now said it does not accept the firm’s findings, claiming a lack of supporting evidence for Second Sight’s view that some of the shortfalls could have been caused by criminals using malicious software, IT system failures or human error.

The Post Office has now published a detailed statement on the issues which says: ‘Investigations over the past three years have confirmed that the Post Office’s Horizon computer system is operating as it should. It is used successfully by 78,000 people to process six million transactions every working day in communities throughout the UK.

‘Both the Post Office and independent forensic accountants Second Sight have found that the majority of the branch losses in complaints put forward to a mediation scheme were, in fact, caused by errors at the counter.

‘The Post Office is concerned that the report by Second Sight, recently made available to scheme applicants for mediation purposes, repeats complaints made by a very small number of former postmasters, as well as a number of assertions and opinions. Second Sight has been unable to demonstrate any evidence to support these.’

Second Sight has so far made no public comment on its report.

The Communication Workers Union (CWU) has put out a statement saying: ‘CWU is not convinced by the claims made by the Post Office in their own investigations which effectively call into question the professional integrity and competence of Second Sight.

'We find it unfair that the Post Office challenges this report whilst having gagged Second Sight so that they cannot defend themselves.’

The Post Office confirmed that there have been nearly 500,000 users of the Horizon accounting system since 2001.