Whistleblower Richard Roll told Panorama in 2015 that postmasters' accounts could be secretly altered
The Post Office threatened and lied to the BBC in a failed effort to suppress key evidence that helped clear postmasters in the Horizon scandal.
Senior managers tried to smear postmasters before Panorama broadcast an interview in 2015 with a Fujitsu whistleblower.
Former insider Richard Roll revealed accounts on the Horizon computer system could be secretly altered.
The Post Office declined to comment while the public inquiry is ongoing.
Mr Roll would go on to play a crucial part in a 2019 High Court case which ruled that bugs could cause errors in the Post Office computer system intended to keep track of transactions in local branches.
Between 1999 and 2015, 700 sub-postmasters and postmistresses - self-employed people who run Post Office branches - were prosecuted for offences such as theft, fraud and false accounting, with some going to prison and others even taking their own lives.
The BBC can reveal that in the period leading up to the broadcast of Trouble at the Post Office, the 2015 Panorama programme featuring the whistleblower testimony:
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