The Post Office threatened and lied to the BBC in 2015 before a Panorama programme with a Horizon whistleblower, the public broadcaster said.
The BBC said experts who were interviewed for the programme were sent intimidating letters by Post Office lawyers who also sent letters to the broadcaster, threatening to sue Panorama.
According to the BBC, senior Post Office managers also told the broadcaster at the time that no staff or the company who developed Horizon, Fujitsu, could access subpostmasters accounts, despite being warned four years earlier this was possible.
The BBC says the claims did not stop the programme, titled Trouble At The Post Office, but it did delay the broadcast of the show.
The Post Office has been contacted for comment. It told the BBC it will not comment while the public inquiry continues.
The Horizon scandal saw more than 700 subpostmasters and subpostmistresses handed criminal convictions after faulty Fujitsu accounting software made it appear as though money was missing at their branches.
Victims have described being shunned by their communities, financially ruined and having their families destroyed.
The overwhelming majority of cases against victims of the scandal were brought by the Post Office using its own private prosecutions powers.
The Post Office public inquiry into the scandal will continue in London on Friday.
The inquiry heard on Thursday subpostmistresses who fell victim to the faulty Horizon IT system accused a Post Office investigator of intimidation and ill treatment.
A former branch manager in Newcastle said she was called “a bitch” while a wheelchair-dependent subpostmistress from Liverpool recalled how she was put into a “tiny parcel lift” to reach an interview room, the inquiry was told.
The investigator in question, Stephen Bradshaw, gave evidence to the inquiry in central London on Thursday and denied acting in any way but professionally throughout the probes he conducted.
A procedural hearing into the appeal of several people against their sentences on Post Office convictions will also take place at Edinburgh High Court on Friday.
It comes as chief executives of Post Office and Fujitsu are set to be questioned by MPs over the Horizon scandal next week.
Parliament’s Business and Trade Committee will meet on Tuesday to examine what more can be done to deliver compensation for victims of what has been labelled the worst miscarriage of justice in British history.
Nick Read, chief executive of the Post Office, and Paul Patterson, head of Europe at Fujitsu, are both due to appear, it has been confirmed.
Prime Minister Rishi Sunak on Wednesday announced that hundreds of the wrongly prosecuted in England and Wales could have their names cleared by the end of the year under blanket legislation to be introduced within weeks.
Pressure on Fujitsu has mounted in recent days, with the Justice Secretary suggesting the firm should repay the “fortune” spent on the scandal if it is found culpable.
Alex Chalk said the Government would want to secure “proper recompense on behalf of the taxpayer” if the “scale of incompetence is as we might imagine”.
The firm has been awarded Government contracts worth billions in recent years and its continued involvement in major IT schemes has raised concerns at Westminster.
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