The Post Office has been accused of being involved in a possible "criminal conspiracy" by an independent investigator who played a pivotal role in uncovering the Horizon IT scandal.
Ian Henderson, who in 2012 alongside Ron Warmington was hired from Second Sight and asked to investigate potential issues with the software, gave evidence at the Post Office inquiry on Tuesday.
Mr Warmington is himself due to give evidence later.
Mr Henderson told the inquiry that he felt former Post Office boss Paula Vennells "frequently and consistently" tried to steer him away from probing miscarriages of justice.
He accused the Post Office of refusing to commission an independent review into the scandal, adding he faced "various threats" of legal action from the company, which appeared to include "defamation, breach of confidence or breach of contract".
Mr Henderson said despite Ms Vennells telling him the Post Office was the "nation's most trusted brand", he believed the company felt it was "above the law".
Second Sight studied over 140 sub-postmasters' claims and produced a report in 2013 finding that the Horizon system was largely "flawed" and that Fujitsu, the creator, and maintainer of Horizon, was altering branch accounts.
However, Second Sight said the Post Office "repeatedly refused" to consider faults with the Horizon system.
An interim report produced by Mr Henderson and Mr Warmington in July 2013, identified two bugs in the 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".