The BBC will publish a report on Thursday written by former judge Lord Dyson, who investigated methods used by Mr Bashir to secure a sit-down chat with Diana that made headlines around the world in 1995.
It is understood the report will be highly critical of senior BBC bosses, with one source telling the Daily Telegraph it “could be the BBC’s phone hacking moment” in reference to the scandal that rocked News of the World in 2011.
Lord Dyson will reportedly conclude that Mr Bashir showed Diana’s brother Earl Spencer fake financial documents to allegedly get access to his sister, a move in breach of BBC editorial guidelines.
Richard Ayre, the BBC’s controller of editorial policy in 1995, told The Telegraph he had informed Lord Dyson in evidence that Mr Bashir would have breached guidelines in mocking up the statements.
He said: “The use of deceit in making factual programmes would have been permissible only in the case of investigating serious crime... and where prima facie evidence of the guilt of that person being investigated had already been obtained.
“Those circumstances clearly don’t apply to an interview with the Princess of Wales.”
The former master of the rolls and head of civil justice was appointed to look into the circumstances surrounding the explosive broadcast, which famously featured Diana saying: “Well, there were three of us in this marriage, so it was a bit crowded”.
Her son, the Duke of Cambridge, welcomed the launch of the investigation late last year, saying it “should help establish the truth behind the actions” that led to the programme.
His brother the Duke of Sussex reportedly also supported the inquiry.
It was launched after Diana’s brother Earl Spencer alleged Mr Bashir showed him fake financial documents relating to his sister’s former private secretary Patrick Jephson, and another former royal household member, and told outlandish and untrue stories about the royal family to gain access to the princess.
Earl Spencer is understood to have provided Lord Dyson with a large dossier of evidence to support his claims, including notes of a conversation he had with Mr Bashir two months prior to the interview with his sister.
Mr Bashir, who was the BBC News religion editor, quit the corporation last week on health grounds after being seriously unwell with Covid-19 related complications. He has previously declined to comment on the inquiry, but said to have mounted a “vigorous defence” of his methods to the probe.
The Independent has approached the BBC for comment.