The Post Office has launched an urgent investigation after it accidently published the names and addresses of 555 postmasters prosecuted during the Horizon scandal.
The company confirmed staff had shared personal details in a document on its website on Wednesday.
The Post Office said it had referred itself to the Information Commissioner’s Office over the breach.
It comes as witnesses continue to give evidence at an inquiry into the Horizon IT scandal, which saw hundreds of sub-postmasters prosecuted for stealing between 1999 and 2015 due to incorrect information from accounting software.
The data breach was first reported by the Daily Mail and led to an angry response from former sub-postmasters, with one telling the paper she was “incandescent”.
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 2019.
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: “We have not received a data breach report on this matter. Organisations must notify the ICO within 72 hours of becoming aware of a personal data breach, unless it does not pose 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.
After years of legal wrangling the government said in January that it would “swiftly exonerate and compensate” those affected.
Two former bosses from Fujitsu, the company behind the Horizon IT system, denied knowing about issues with the system at the inquiry on Wednesday.
One of them, Duncan Tait, said Post Office bosses “never escalated to me any issues regarding Horizon integrity”.
“Indeed, I heard repeatedly that the subpostmasters’ claims regarding Horizon integrity were unfounded and that the system was working well.”
