The BBC is investigating the tactics used to obtain an interview with the late Princess Diana where she uttered the tragic statement about her marriage to Prince Charles: "There were three of us in this marriage".

The interview between journalist Martin Bashir and Diana aired in 1995 but 25 years on, new evidence suggests that the reporter may have used forged bank statements and other unethical methods to convince Diana to agree to the interview.

The incident is now being properly investigated, but it's complicated as Bashir, 57, is now gravely ill with COVID-19.

Over the weekend, the BBC apologised to Princess Diana's brother Charles Spencer after he brought forward the evidence.

The network admitted that Bashir showed Spencer bank statements doctored by a staff graphic designer. Spencer had alleged that Bashir told his sister “fantastical stories to win her trust” and showed him fake bank records that reportedly helped land Bashir the interview.

Charles said he found a letter sent to him by Bashir that brought up rumours of an affair between Charles and the family nanny.

At the time, Diana was allegedly deeply worried she was being spied on and Bashir's "evidence" made her confident to do the interview.

“There were three of us in this marriage, so it was a bit crowded,” Diana said in the interview, referring to Charles’ affair with Camilla Parker Bowles.

Weeks after Diana uttered that sentence, the royals began divorce proceedings.

“Suggesting that mocked-up documents were genuine was wrong then and it’s wrong now; the BBC of today is happy to apologise for this. The BBC’s editorial processes are now even tougher and this would not happen today,” said a statement from a BBC spokesperson sent to the New York Post. “The BBC’s records say that the Princess of Wales said she hadn’t seen the mocked-up documents and they had played no part in her decision to take part in the interview.”

The BBC is now investigating the interview.

“The BBC has apologised. We are happy to repeat that apology. And while this was a quarter of a century ago, we absolutely will investigate – robustly and fairly – substantive new information,” a spokeswoman told Deadline. “We have asked Earl Spencer to share further information with the BBC.

“Unfortunately, we are hampered at the moment by the simple fact that we are unable to discuss any of this with Martin Bashir, as he is seriously unwell. When he is well, we will of course hold an investigation into these new issues.”