The BBC announced Wednesday the immediate start of an investigation into how it obtained an explosive 1995 interview with Princess Diana that lifted the lid on her troubled marriage to Prince Charles.

The corporation said it had approved the appointment of former Supreme Court judge John Dyson to lead the probe, which comes after calls from the late princess' brother, Charles Spencer.

Spencer has alleged that the flagship "Panorama" programme reporter who conducted the interview, Martin Bashir, showed him faked documents to persuade his sister to take part.

In the November 1995 interview, which was watched by a record 22.8 million people, Diana detailed her collapsing marriage to the heir to the throne.

She famously said "there were three people" in her marriage -- her, Charles and his long-time lover Camilla Parker-Bowles -- and also revealed she had been unfaithful.

Diana and Charles formally divorced in 1996. She died in a Paris car crash the following year.

New reports have surfaced alleging that Bashir used underhand methods to persuade Diana to talk, including by claiming her own staff members were being paid to spy on her.

"This is an important investigation which I will start straight away," Dyson said in a statement. "I will ensure it is both thorough and fair."

The BBC set out the terms of reference for the probe, focusing principally on the role of Bashir, who was little-known at the time but went on to have a global career.

It will consider "the mocked-up bank statements purporting to show payments to a former employee of Earl Spencer... (and) the purported payments to members of the Royal Household".

It will also look at revelations made by Spencer this month about Bashir to the Daily Mail, which said the reporter made "lurid claims" about Queen Elizabeth II, Charles and other royals.

Bashir has not responded to the latest claims. The BBC has said he was seriously unwell after contracting the coronavirus.

The BBC has been accused of a cover-up in a previous inquiry when rumours about Bashir's alleged methods first surfaced.

The publicly funded broadcaster said its latest investigation will be published once completed.

"The BBC is determined to get to the truth about these events and that is why we have commissioned an independent investigation," director-general Tim Davie said.