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The Post Office has apologised after it published the names and addresses of 555 postmasters it pursued during the Horizon scandal.

The company confirmed personal details had been shared in a document on its website and said it had referred itself to data watchdog the Information Commissioner’s Office.

One former sub-postmaster said the breach had caused "upset, distress and anger" and added the data leak was "preventing people from trying to heal".

In response, chief executive Nick Read apologised and said the leak was "a truly terrible error".

It comes as witnesses continue to give evidence at an inquiry into the scandal, which saw hundreds of sub-postmasters prosecuted for theft between 1999 and 2015 when incorrect information from Horizon accounting software made it look like money was missing from their branches.

Other sub-postmasters which were not prosecuted were pursued by the Post Office over alleged losses in accounts.

The data breach on Wednesday was and led to an angry response from former sub-postmasters.

Former sub-postmaster Christopher Head he had written to Mr Read and Post Office chair Nigel Railton.

He wrote: "As you can imagine this has caused a great amount of upset, distress and anger amongst those whose data is now within the public domain."

Many sub-postmasters who "hadn't shared details with their own families" and others who were "extremely traumatised by this whole scandal even today" had been hit by the breach, Mr Head wrote.

He later told the 91�ȱ� that many sub-postmasters were suffering from post-traumatic stress and "trying their best to move on".

"The problem is that we are getting new scandals within a scandal and new revelations on an almost daily or weekly basis that is preventing people from trying to heal," he added.

In response to Mr Head's letter, Mr Read said: "This is a truly terrible error and one for which at this stage I can only apologise."

The Post Office is "reviewing with urgency the right follow-up actions to take" for those people who have been affected by the breach, he added, and how to correct the organisation's processes.

One former sub-postmaster told the Daily Mail she was “incandescent”. Wendy Buffrey said that the action could "destroy lives" because criminals could now target sub-postmasters who had received compensation.

Ron Warmington, the forensic investigator whose firm Second Sight was brought in to probe the Horizon system in 2013, told the Mail it was “an extraordinary breach” of confidentiality and “another example of Post Office incompetence”.

The leaked document contained the names of 555 former subpostmasters who sued the Post Office in 2017.

In 2019, the firm agreed to pay them £58m in compensation, but much of the money went on legal fees.

In a statement the Post Office said the document had been removed from its website.

It said: “We are investigating as an urgent priority how it came to be published. We are in the process of notifying the Information Commissioner’s Office (ICO) of the incident, in line with our regulatory requirements.”

An ICO spokesman said: “Post Office Limited have made us aware of an incident and we are assessing the information provided."

Organisations need tell the watchdog about a data breach within 72 hours of becoming aware of it, if it poses "a risk to people’s rights and freedoms".

More than 900 sub-postmasters were prosecuted for stealing because of incorrect information from Horizon in what has been called the UK's most widespread miscarriage of justice.

Many sub-postmasters went to prison for false accounting and theft, and many were financially ruined.