Diana’s interview with journalist Martin Bashir, watched by more than 20 million viewers in Britain, shocked the nation when she admitted to an affair and gave other intimate details of her failed marriage to heir-to-the-throne Prince Charles.

Last November, 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.

“In recent months the Metropolitan Police Service received correspondence alleging unlawful activity in connection with a documentary broadcast in 1995,” Commander Alex Murray said in a statement.

He said the issue had been assessed by specialist officers and they had been given extensive legal advice.

“Following this detailed assessment and in view of the advice we received, we have determined that it is not appropriate to begin a criminal investigation into these allegations. No further action will be taken,” Murray said.

The police decision comes days before an interview between Diana’s youngest son, Prince Harry, and his wife Meghan with U.S. talk show host Oprah Winfrey is due to be broadcast.

That interview has already caused a crisis in the royal family before it has been aired, amid allegations of bullying against Meghan and her counter claims Buckingham Palace has been “perpetuating falsehoods”.

Diana, who died in a Paris car crash in 1997, made her first public comments about her doomed marriage in the 1995 interview, but her brother said it had been secured after he was shown false bank statements amid other dishonest practices.

His comments prompted the BBC to announce a former UK Supreme Court judge would lead an inquiry into how the broadcaster secured the interview and whether executives covered up any wrongdoing, a move welcomed by Harry and his elder brother Prince William.