The Post Office threatened and lied to the BBC in 2015 before a Panorama programme with a Horizon whistleblower, the broadcaster has said.
The corporation said experts who were interviewed for the programme were sent intimidating letters by Post Office lawyers who also sent letters to the BBC, threatening to sue Panorama.
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 that this was possible, according to the BBC.
The claims did not stop the programme, titled Trouble at the Post Office, but it did delay the broadcast of the show, the corporation said.
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 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 inquiry heard on Thursday that subpostmistresses who fell victim to the faulty Horizon IT system accused a Post Office investigator of intimidation and ill treatment.
The investigator in question, Stephen Bradshaw, gave evidence to the inquiry in central London and denied acting in any way but professionally throughout the probes he conducted.
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