The Post Office is calling for evidence from people that have worked with the Horizon accounting system as it takes a closer look at claims that agents have been wrongly accused of false accounting.
The Post Office wants people to come forward without fear of reprisal.
Earlier this month Computer Weekly reported that the Post Office had reconsidered its stance on the Horizon system, which was developed by Fujitsu, and was working with the Justice for Sub-postmasters Alliance (JFSA) to look closer at the allegations.
Independent forensics expert Second Sight is carrying out the investigation.
This is a marked change in attitude from the Post Office. Despite numerous complaints, the Post Office has consistently stated here is no fault with the Horizon system.
In its latest Branch Focus internal update to branches, the Post Office has called on people to come forward with information.
“As you may have heard, Post Office is working with independent forensic experts, Second Sight and Justice for Sub-postmasters Alliance, to review reported concerns that have been raised about the Horizon system," said the Post Office.
"The Post office and JFSA have signed an agreement to ensure that any agent, contractor or employee with any evidence can feel free to report their observations.”
The agreement ensures evidence can be given without fear of any comeback from the Post Office, said Alan Bates, chairman of the Justice for Sub-postmasters Alliance (JFSA).
The Post Office has asked for submissions before 28 February this year.
Dozens of postmasters have been charged and even jailed for accounting shortfalls. Others have had to make up cash discrepancies following prosecutions. Some Post Offices have been forced to close.
In 2009, Computer Weekly interviewed some of those affected. The sub-postmasters related stories of bankruptcy, prosecution and disrupted livelihoods.
Computer Weekly is still being contacted by sub-postmasters on the issue. Bates at the JFSA said: “It is still early days but people are beginning to come forward with evidence.”
He also called for anybody who worked on the development of Horizon to come forward if they have any evidence.