The comment comes as over 800 post offices complained a software issue was causing till systems to freeze or run slow in branches last week.

A PO spokesperson told betterRetailing: “The intermittent slow-running of the Horizon system which affected some Post Office branches has now been fixed. We have apologised to our postmasters for the inconvenience this caused.”

However, the issue has made partnered stores doubt the viability of the Horizon system. Vince Malone, owner of Tenby Premier Store & Post Office in Pembrokeshire experienced the problems for five weeks. “It has made me question the robustness of the whole system,” he said.

A letter seen by betterRetailing from the PO’s head of IT services Martin Godbold first revealed the problem. The Communication Worker’s Union was sent the letter and then shared it with betterRetailing.

“I know this is frustrating as there seems to be no pattern to the issue – so far we haven’t been able to identify a specific time, a specific counter or a specific transaction that is the cause,” he said. “We want to make sure any fix we roll out will improve the situation for all.”

The IT head revealed the PO’s so-far unsuccessful attempts to resolve the issue stating: “Over the past three weeks, we have been working with our technology partners to try to identify what may have caused this issue so we can then fix it.”  He continued: “This has meant working backwards to see if any of the changes we have rolled out into the network over the past few months have caused the intermittent slow running; and to monitor branches in real-time to see if there are any hardware issues.” 

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In the letter, Godbold urged subpostmasters to report any issues they experience to its IT service desk.

Baker added that the issues are likely to cause longer queues instore and dissatisfaction among customers.

The new issue comes as the Post Office faces growing pressure and legal costs over alleged historic issues with the Horizon system. A high court case led by 550 subpostmasters and post office staff claim Horizon glitches led the Post Office to falsely accuse them and others of fraud, leading to imprisonment, bankruptcies and criminal convictions.

CWU’s national branch secretary Mark Baker told betterRetailing that the number of subpostmasters affected could in fact be higher than Godbold stated. “The Post Office have admitted in a communication that around 800 branches have reported slow running to them,” he said. “This figure will be higher because many postmasters will not contact the IT help desk as they have lost all faith in it, and the time it takes to get them to understand the problem.”

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“The slow running seems to have caused discrepancies to postmasters’ balances due to the temptation to press the screen when it freezes, which could be sending transactions more than once,” he said.