The BBC has been urged to end its "cover-up" and release a letter it received from the Palace just days before airing the Princess Diana Panorama interview.
The document, which has remained hidden in BBC archives for three decades, was sent to the then-director general John Birt four days before the 1995 broadcast.
The existence of the letter, which is understood to have come from Queen Elizabeth II's office, was confirmed in 10,000 pages of heavily redacted documents released by the BBC last week.
The BBC was ordered, under freedom of information laws, to share internal emails that could uncover how senior executives behaved when they discovered Bashir's actions.
Princess Diana sat down for a 1995 interview with Martin Bashir
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On Saturday, historians and campaigners called on the BBC to end its "cover-up" and release the letter from Buckingham Palace.
Lord Birt received the letter on November 16, 1995, according to reports in The Mail.
The newly released files refer to the "Letter from Palace to DG – withheld in full" and the date, but not the document itself.
The BBC has faced criticism for obstructing freedom of information requests about the Panorama scandal.
Princess Diana being interviewed by Martin Bashir for PanoramaPAUnder Freedom of Information laws, public bodies can refuse to release communications with the Royal Family under a Section 37 exemption.
But Freedom of Information campaigners and the Information Commissioner's Office claimed the BBC was not obliged to cite the exemption.
Maurice Frankel, of the Campaign for Freedom of Information, told The Mail: "[The BBC] are free to disclose this letter from the Palace should they wish, and I think they should. It's a matter of real public interest."
A BBC spokesman added: "We take our responsibilities... under the Freedom of Information Act extremely seriously.
Lord Birt received the letter from the Palace in November 1995
PA
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"This specific exemption covers correspondence with the Royal Household, recognising the need for all parties to have a 'safe space' to ensure a free and frank exchange of information."
After the Panorama interview was broadcast in 1995, the late monarch removed the BBC's exclusive rights to the production of her annual Christmas broadcast, which was viewed as an act of revenge.
The Panorama episode was watched by over 20million viewers. Subsequently, Queen Elizabeth II wrote to the then-Prince and Princess of Wales, urging them to divorce.
In 2021, an official inquiry by High Court judge Lord Dyson concluded that Bashir had faked bank statements to convince Princess Diana that members of her inner circle were selling details of her private life and that the BBC covered up what it knew about the journalist's activities.