In Andrew Morton's controversial biography, Diana: Her True Story, first published in 1992, Diana recalled "one of her bravest moments" - a calculated confrontation with Camilla at a birthday party for the Duchess' sister.
The 1989 incident came after Diana realised Charles and Camilla had reignited their relationship.
Turning up to the event unannounced and uninvited, Diana interrupted a conversation between Camilla, Charles and another male guest.
Morton claims nobody knew Diana was going to show up, and in a truly rebellious act, the princess interrupted the conversation, noting Camilla looked "really uncomfortable and put her head down."
"OK, boys, I'm just going to have a quick word with Camilla and I'll be up in a minute," Diana said.
The late princess later recalled the two men "shot upstairs like chickens with no heads," adding, "I could feel, upstairs, all hell breaking loose."
Diana recalled in a taped conversation, "The voice inside me had said, 'just go for it'."
"I said, 'I know what's going on between you and Charles and I just want you to know that'."
The princess noted Camilla's reply was "very interesting".
Diana told Morton, "She said to me, 'You've got everything you ever wanted. All the men in the world fall in love with you and you've got two beautiful children, what more do you want?'"
Diana replied, "I want my husband."
She finished the confrontation by telling Camilla to not "treat her like an idiot."
Following the encounter, Diana said of Charles: "He was over me like a bad rash," adding, "I cried like I have never cried before."
In frustration, Diana began her own string of extramarital affairs until she and Charles eventually separated, then divorced at the behest of Queen Elizabeth II in 1996.
The couple managed to stay civil while co-parenting their sons until Diana's tragic death in August 1997.
Prince Charles and Camilla wed in 2005.