This month, her brother Charles Spencer said the BBC had failed to apologise for what he said were forged documents and "other deceit" which led him to introduce Diana to journalist Martin Bashir.

During her interview with Bashir, watched by more than 20 million viewers in Britain, Diana shocked the country by admitting to an affair and giving intimate details of her failed marriage to heir-to-the-throne Prince Charles.

The BBC's Director General Tim Davie said the broadcaster was determined to get to the truth about Spencer's claims.

"This is an important investigation which I will start straight away," said John Dyson, the former Supreme Court justice appointed to lead the inquiry.

"I will ensure it is both thorough and fair."

Spencer says Bashir made a series of allegations to himself and his sister such as that Diana was being bugged by the security services and that two senior aides were being paid to provide information about her.

He says Bashir provided fake bank statements to back up the claim.

Others involved in making the program have also come forward to say the BBC had covered up wrongdoing.

Bashir has made no public comment to media and the BBC says the journalist - who gained global renown from the Diana interview and is currently the corporation's religious affairs correspondent - is currently on sick leave, recovering from heart surgery and from contracting COVID-19.

The BBC said its investigation would examine five questions such as what steps the BBC and Bashir took to obtain the interview, "including (i) the mocked-up bank statements purporting to show payments to a former employee of Earl Spencer (ii) the purported payments to members of the royal households and (iii) the other matters recently raised by Earl Spencer".

Australian Associated Press