The founder of an action group for ex-subpostmasters accused of theft, fraud or false accounting has welcomed a Post Office investigation into the computer system used in all its outlets.
Alan Bates, of Old Colwyn, Conwy, says he formed the Justice For Subpostmasters Alliance based on his experience as a subpostmaster in Wales.
The Post Office has appointed external investigators to examine alleged deficiencies in the system.
It denies Horizon has problems.
Mr Bates claimed the Post Office terminated his contract in 2003 because he was "making a fuss" about the Horizon system.
Mr Bates said dozens of former subpostmasters from Wales have turned to the alliance for help.
It claimed many had suffered an injustice, even though some have pleaded guilty to false accounting for thousands of pounds of losses.
Mr Bates said he put their concerns at the feet of the Horizon accounting system.
He said: "Ever since it's been installed, it's been nothing but trouble and it's caused enormous upset and grief and hardship to people across the whole of the country.
"We don't think it's accurate. We think there are flaws in the system.
"Fortunately, now, it looks like the Post Office are going to look in to some parts of it, not the whole issue. At least it's a start."
Mr Bates said he had raised problems about Horizon ever since it was installed.
He said his previous experience with similar accounting systems had helped him to troubleshoot problems with his Post Office balances when they cropped up.
On one occasion, his accounts showed his till was more than £5,000 short of the money expected, he said.
Working back through the till receipts for the whole week, he said he found more than £5,000 had appeared as duplicate transactions.
"This had come about because of a software upgrade they had installed that week," said Mr Bates.
He added: "One of the big problems with Horizon is the inability to fully examine all the data you have put in the system.
"You were not allowed to interrogate it. They restricted the access.
"I refused to be held liable for a system that I and my staff were unable to access to check."
The Post Office has instructed a firm of forensic accountants, 2nd Sight Limited, to conduct an independent review of 10 existing cases raised by a number of MPs and the law firm Shoosmiths.
About 100 sub-postmasters and sub-postmistresses have registered an interest in suing Post Office Ltd over its Horizon computer system.
Horizon records financial transactions in branches across the UK.
The Post Office has repeatedly denied there is a problem with the system, saying the claims have been made by a very small number of people who had contracts with it.
It would not comment on Mr Bates's case or allegations.
A Post Office spokesperson said: "The Post Office continues to have absolute confidence in the robustness and integrity of its branch accounting processes.
"Over the past 10 years, many millions of branch reconciliations have been carried out with transactions and balances accurately recorded by more than 25,000 different subpostmasters.
"The Post Office has no hesitation in agreeing to an external review of these few individual cases that have been raised with us by a number of MPs."