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 Bashir.
The BBC's director-general Tim Davie said the broadcaster was determined to get to the truth about Spencer's claims.
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 programme 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 the corporation's religious affairs correspondent, is now on sick leave, recovering from heart surgery and from contracting Covif-19.
The BBC said its investigation would examine five questions such as what steps the BBC and Bashir took to obtain the interview, “including:
• The mocked-up bank statements purporting to show payments to a former employee of Earl Spencer
• The purported payments to members of the royal households and;
• The other matters recently raised by Earl Spencer.”
Reuters