The latest excerpt of a biography of Camilla, Duchess of Cornwall, lifts the lid on the ill-fated royal marriage

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Prince Charles was the “architect of the disaster” of his marriage to Princess Diana, a new book claims.

Even though he had ended his relationship with former girlfriend Camilla Parker Bowles as soon as he decided to get engaged to Diana in February 1981, the young princess-to-be needed more reassurance, The Duchess: The Untold Story, by Penny Junor claims.

In the newest excerpt of the book posted on the Daily Mail on Monday, Junor unveils a catalogue of mistakes in the marriage that rocked the British monarchy in the ’90s. The new revelations come ahead of Camilla, Duchess of Cornwall’s 70th birthday next month and just before many of Diana’s fans around the world mark the 20th anniversary of her death in August.

According to the book, Diana resented Charles for sitting for hours painting watercolors during their honeymoon onboard the HMS Britannia, so one day she destroyed his painting and his equipment.

On another occasion, Junor claims the royal couple were consulting their schedules when a photograph of Camilla fell out of Charles’ diary. Later, Diana noticed the prince was wearing a pair of gold cufflinks engraved with interwoven Cs — which she took to mean Charles and Camilla.

Princess Diana and Prince Charles in 1981
| Credit: Central Press/Getty

“It’s hard to believe that anyone as intelligent and well-read as the Prince of Wales could be so stupid — so utterly incapable of imagining what a new wife might conclude if her husband carried a photograph of his old girlfriend in his diary,” Junor says of the cufflink incident, according to the new except.

Prince Charles
Princess Diana and Prince Charles in 1992
| Credit: Anwar Hussein/Getty

Diana started suffering from bulimia during the engagement and it continued in the early days of the marriage. Charles “was way out of his depth . . . had no idea how to treat a wife, let alone one who was secretly making herself sick several times a day,” Junor says.

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A close friend of Charles tells Junor of his marriage to Diana, “He made a huge mistake. You can sympathize with Diana — oh God, yes. Put that way, he was the architect of the disaster . . . also he wouldn’t have had the sensitivity.

Prince Charles at Polo
Prince Charles and Camilla, Duchess of Cornwall, before his marriage to Diana
| Credit: Hulton-Deutsch Collection/CORBIS/Corbis via Getty

“He’s very interested in objective things, but not subjective, so he couldn’t have understood the complexities of her feelings.”

Charles And Diana Unhappy
Princess Diana and Prince Charles
| Credit: Tim Graham/Getty

Diana and Charles’ marriage soon broke down after that.

The Royal Wedding of HRH Prince Charles and Mrs. Camilla Parker Bowles - The Blessing Ceremony - Inside
Prince Charles and Camilla, Duchess of Cornwall, at their wedding
| Credit: Anwar Hussein Collection/ROTA/WireImage

Camilla and Charles renewed their affair in 1986 after friends urged the prince to reconnect with Camilla after they grew worried about his happiness and possibly heading towards a nervous breakdown. According to friends, Camilla was “the only person who might be able to lift his spirits,” Junor claims.

Diana, for her part, had already started her affair with cavalry major James Hewitt, Junor says.