Princess Diana called out Camilla Parker Bowles at her sister's 40th birthday party after discovering Prince Charles 'had gone back to her'.
The pair had it out as Diana confronted her, telling the now Duchess of Cornwall she knew what was going on at a party held for Lady Annabel Goldsmith in February 1989.
Andrew Morton’s book Diana: Her True Story — In Her Own Words marks the 25 years since her tragic death.
In one explosive chapter, it tells of the face-to-face with Camilla as she detailed the misery of her marriage and her struggle with bulimia.
In the book, Diana says it worst day of her life when she realised that Charles and Camilla were still very much an item.
After plucking up the courage to ask her rival for a 'quick word' she let it all out.
"I’m sorry I’m in the way - I obviously am in the way and it must be hell for both of you, but I do know what is going on. Don’t treat me like an idiot," she told her, in extracts of the book being serialised by the Daily Mail.
Not even Camilla's protest, saying she had no idea what she was talking about, could turn Diana's head.
"I know what’s going on between you and Charles, and I just want you to know that," she added.
After being told 'it’s not a cloak-and-dagger situation’ the Princess was allegedly told by Camilla: "You’ve got everything you ever wanted - you’ve got all the men in the world falling in love with you, and you’ve got two beautiful children. What more could you want?"
Diana simply replied that all she wanted her husband.
When Morton published his biography Diana in 1992, it was condemned by the Archbishop of Canterbury.
He faced so much criticism one MP suggested he should be banished to the Tower of London.
But he defends his writing as Diana's truth, and while he dismissed conspiracy theories surrounding her death, he adds she was "absolutely" being conspired against generally.
Journalist Mr Morton this weekend revealed that Charles would circle TV shows in the Radio Times to fool his wife into believing he was planning on staying in, hiding an affair.
The royal biography said: "Charles would circle a TV programme with his pen in the Radio Times for the evening to make her think he was at home, when actually a Ford Escort estate car had driven him 12 miles to meet Camilla Parker Bowles."
He added: "At the time, everybody still believed it was the perfect marriage – the handsome prince and his beautiful young bride. But nothing could have been further from the truth.
"One particular part of the tapes from 1991 which still gets the hairs standing up on the back of my neck was when she said she wanted to go to Paris for a weekend to walk along the pavement and not have anyone follow or recognise her."
