There was "possibly a criminal conspiracy" at the Post Office, according to an independent forensic accountant drafted in to investigate the controversial Horizon accounting system.
Ian Henderson, one of the two forensic accountants from Second Sight paid by Post Office (POL) to review sub-postmaster convictions involving Horizon in 2012, told the public inquiry into the scandal that former chief executive Paula Vennells "frequently and consistently" tried to steer him away from probing miscarriages of justice.
Mr Henderson explained how he had signed a non-disclosure agreement (NDA) with Post Office and claimed he later faced a "thinly veiled threat" from the company's then head of legal Chris Aujard "to bankrupt me if I continued causing trouble".
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A 2013 interim report produced by Mr Henderson and his colleague Ron Warmington identified two bugs in the Horizon system that caused problems for 76 branches.
The forensic accountants were sacked in 2015, and Mr Henderson said he believed they were dismissed because they were "getting too close to the truth".
He told the inquiry he felt Post Office was "constantly sabotaging our efforts to seek the truth irrespective of the consequences".
Counsel to the inquiry Jason Beer KC asked: "What had happened to the 'shared desire to seek the truth irrespective of the consequences?'"
Mr Henderson replied: "I think we'd moved on from that.
"I'd formed the view that quite early on in the process, Post Office was getting advice from external lawyers about the financial consequences of what we were finding - the fact that they might be looking at very material amounts of compensation."
He added: "It was very clear that Post Office senior management were very concerned about the public perception, the brand image - I mean, Paula Vennells in meetings was very open about it.
"She was determined to promote the brand of Post Office."
Legal 'threats' under NDA
In his witness statement to the inquiry, Mr Henderson said he felt Second Sight was dealing with a cover-up.
He said: "By February 2015, I no longer had confidence that POL was taking our concerns seriously or dealing with them in an appropriate manner.
"I felt we were dealing with a cover-up by POL and possibly a criminal conspiracy.
"I was concerned about the various threats that had been made to me by POL concerning alleged breaches of my NDA and my duties of confidentiality.
"Accordingly, I had to find a way of communicating my concerns, but which limited the risk of a legal action against me, or Second Sight, by POL.
"The most likely threats appeared to be an action for defamation, breach of confidence or breach of contract."
He continued: "My work for POL and the (mediation) Scheme was probably the most challenging in the 40 years of my career as a chartered accountant.
Second Sight 'steered away' from truth
"One of the reasons it was challenging was that POL would say one thing in public, and then do something different in private.
"An example of this was Paula Vennells' statement to the Parliamentary Select Committee in February 2015, that our work had found 'no evidence of miscarriages of justice' and 'it was important that we surface any miscarriages of justice'.
"Paula Vennells frequently and consistently attempted to steer Second Sight away from investigating potential miscarriages of justice.
"When I first met Paula Vennells, she told me that POL was the nation's most trusted brand with a history of over 400 years.
"As our work continued, I increasingly formed the view that because of this history, POL somehow felt it was above the law.
"I formed the view that POL was constantly sabotaging our efforts to seek the truth irrespective of the consequences.
"Requests for documents were either ignored or responses were excessively delayed.
"Unjustified claims of legal professional privilege were used to justify withholding documents from us."
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Concluding his witness statement, Mr Henderson added: "We tried to go where the evidence took us, but increasingly we were finding evidence of questionable conduct by POL, some of which, in my opinion, was probably criminal.
"In the course of our work, I increasingly felt that our overriding duty was, in a phrase attributed to Alan Bates, to help 'the skint little people' who didn't have a voice and had been so badly treated by POL."
In her own evidence to the inquiry last month, Ms Vennells said she had been perhaps "too trusting" of people around her when it came to getting to the truth about miscarriages of justice.
More than 700 sub-postmasters were wrongly convicted of charges including theft and false accounting between 1999 and 2015 and many are still awaiting compensation.
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