The former Post Office chief executive Paula Vennells repeatedly broke down in tears as she told a public inquiry that she had been misled by her staff about the safety of the prosecutions of branch operators.

Under questioning from Jason Beer KC, the inquiry’s lead counsel, about her knowledge of the faults in the Horizon IT system, which led to hundreds of people being wrongly persecuted over missing funds, Vennells claimed: “I was too trusting.”

Text messages were also shared with the inquiry revealing that the former Royal Mail boss Moya Greene messaged Vennells, 65, in January this year accusing her of being part of a cover-up of wrongful prosecutions.

“I don’t know what to say,” wrote Greene, who headed Royal Mail between 2010 and 2018 and was Vennells’ boss until the Post Office was split off in 2012. “I think you knew.”

Vennells, who led the Post Office between 2012 and 2019, responded: “No Moya, that isn’t the case.”

Between 1999 and 2015, hundreds of post office operators were prosecuted on the basis of faulty accounting software, and thousands more were bankrupted or forced to pay back cash.

In her evidence, during which she broke down on four occasions, Vennells claimed the Post Office’s structure and the decisions of some employees not to pass on information, including legal advice and damning independent reports, meant she was unaware that people were being wrongly prosecuted or chased for missing funds.

Vennells, who repeatedly insisted during her tenure that the Horizon system was unimpeachable despite mounting evidence to the contrary, was forced to deny that she had prioritised the reputation of the Post Office over the lives of branch operators.

In one emotional moment, Vennells was confronted with emails in which she had asked staff whether they could find out if “there were previous mental health issues, and potential family issues” that could explain the suicide of Martin Griffiths, 58, who had been pursued by the Post Office in 2013 for tens of thousands of pounds that had gone missing from his branch.

“You had just been told about his death and you were trying to get on the front foot here, weren’t you?” Beer asked.

“What I was trying to do, quite simply, it was to get the wider picture,” Vennells responded.

She was also questioned over a 2015 Post Office briefing paper that advised her to deny to a Commons select committee that the Horizon system could be remotely accessed by its maker, Fujitsu, and admit to it only if “pushed”.

Asked why she had been given this advice by her press office, Vennells told the chair of the inquiry, the former high court judge Sir Wyn Williams: “Maybe other people knew more than I did and they were trying to direct me in a certain way.”

Other evidence showed that Vennells had been told in 2011 in an IT audit report that remote access was possible. She said she had not understood what she had been reading. The revelation in 2019 that remote access was possible paved the way for the exoneration of convicted post office branch operators.

Vennells, who was stripped of her CBE in February after the scandal, had appeared composed when she first took the oath at the inquiry, being staged at Aldwych House in central London, but her voice failed her when confronted with evidence that she had misled MPs during a meeting in June 2012.

She had told a delegation, including Lord Arbuthnot, who was a key campaigner for the victims of the scandal, that there had not been a single case prosecuted where Horizon had been found to be at fault.

Beer listed three cases that had gone to court by that stage in which those accused had blamed the accounting system and had been acquitted.

“You know it’s incorrect,” Beer said.

Breaking down in tears, Vennells responded: “I didn’t know that and I’m incredibly sorry.”

She was also presented with an email exchange from October 2013 in which she had described detailed accounts of the cases of post office branch operators as “very disturbing”.

Beer asked how it was that she subsequently changed her mind and concluded that the Horizon system’s faults did not have any role to play in the loss of branch funds.

Vennells said the cases had been explained away or they had been going through the criminal cases review.

Vennells further claimed she had been unaware until 2012, when she became chief executive after five years in roles as network director and managing director, that the Post Office carried out its own criminal investigations and prosecutions of staff.

Beer questioned Vennells on why she had failed to respond to a request from the inquiry in her nearly 800 pages of written evidence to reflect on the mistakes she had made as chief executive.

“Were you adopting a wait and see approach?” asked Beer. “Let’s see what comes out in evidence. See what I’ve got to admit to. And then I’ll admit that.”

Vennells responded: “My memory was not very good to begin with this process. It has improved as I’ve gone through the documentation, that was important. And by the time I got to December last year, when the draft went in, I had simply run out of time to answer these questions properly.”

Vennells, who campaigners say should be under criminal investigation, was warned by the inquiry’s chair that she had a right not to incriminate herself during evidence.

The Met police are a core participant in the inquiry and a spokesperson said they were monitoring the evidence remotely.