Princess Diana and Camilla Parker Bowles had a heated confrontation at the latter’s sister’s birthday party. But it has been revealed that the Princess of Wales wasn’t supposed to be there in the first place.

In the book “The Duchess Chronicles,” royal author Tina Brown said that Camilla’s sister, Lady Annabel Goldsmith, gave the Prince and Princess of Wales an invitation to her bash. But only Prince Charles was expected to attend.

“Charles had never expected Diana to attend what she had already referred to as a ‘ghastly’ party. Like Camilla, Annabel would have expected Diana to understand that the ‘and Princess of Wales’ on this particular invitation was strictly a matter of form and should be tactfully declined,” Brown said.

But to everyone’s surprise, Princess Diana decided to attend the party. At first, Prince Charles tried to convince his wife otherwise, but Princess Diana wouldn’t budge.

Princess Diana’s former protection officer, Ken Wharfe, said that the Prince of Wales needled Princess Diana all the way about her decision to suddenly attend the celebration. And following Princess Diana and Camilla’s confrontation, the late royal declared that it was one of the bravest moments of her life since she married Prince Charles.

Meanwhile, Princess Diana reportedly had a lonely and miserable life inside the palace. Royal author Andrew Morton told Fox News that he decided to write “Diana: Her True Story” to help the Princess get her story out.

“This kind of media cut-out… she felt like she was enduring a lonely miserable life inside the palace and outside, she was adored… It was incredibly frustrating as far as she was concerned because everyone still believed in the fairytale… And she knew it was a nightmare,” he said.

Princess Diana wed Prince Charles in 1981. Back then, she already knew that the future King was having an affair with Camilla. But Prince William and Prince Harry’s mom still pushed through with the wedding because she loved him.