Sir Vince said he was aware of more general issues with the Post Office at the time.

In his first meeting with then-Post Office chief executive Paula Vennells, he said he had “a spirited discussion when I raised the treatment of postmasters”.

He added in his witness statement that the Post Office was “authoritarian” and “dealt with us in an arrogant way”.

He also said in the statement that he agreed with former sub-postmaster Sir Alan Bates’s description of Post Office middle management as “thugs in suits”.

Sir Vince described the Post Office at the time as a “monopoly”, which he wanted to change.

He said he had wanted to “address the imbalance” between the Post Office and sub-postmasters by creating a “mutual structure”, which would have in effect given sub-postmasters control of the organisation.

He said he raised some of these issues with Ms Vennells but the mutualisation “unfortunately never came to fruition”.

Instead, the Post Office was split from the Royal Mail group under Sir Vince’s watch and remains a government-owned company.

Despite raising these general concerns, Mr Beer questioned why Sir Vince was not aware of serious problems with Horizon until 2015.

A letter from 2012 signed by Sir Vince to then-Labour leader Ed Miliband said he “remains fully confident about the robustness and integrity” of the Horizon software.

It was written in response to Mr Miliband’s concerns about a sub-postmaster. Sir Vince said he never saw the letter, despite it being written on his behalf.

“The problem was there were about several hundred letters, and emails, [that] would come in every day,” he said, adding that he was on international visits at the time.