King Charles responds to Charles Spencer's claims about Princess Diana in a rare statement

King Charles responded to claims made about him in Charles Spencer's upcoming book in an unusual move.

The Daily Mail published an excerpt of Spencer's upcoming memoir, "Swan Song: Diana, My Sister," Wednesday, in which the king's former brother-in-law alleged a heated conversation he had with Charles after the death of his sister and the former Princess of Wales, Princess Diana.

"Prince Charles continued his stream of anger, without me interrupting. Then he stopped. I assumed the pause signaled an effort to bring himself under control, but then he unleashed down the phone what I have always taken to be his pent-up contempt for Diana, and his horror that the world was so bowled over by her death," Spencer alleged, according to the Daily Mail. "‘Rest assured,’ he hissed, ‘we’ll forget her soon enough!’"

According to the excerpt, the disagreement between King Charles and the 9th Earl of Spencer was regarding the decision to have Prince William and Prince Harry, then 15 and 12, walk behind their mother's casket during the funeral procession.

King Charles responds to claims made by Charles Spencer in his new memoir. (Getty Images)

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A Buckingham Palace spokesperson issued a rare statement on behalf of the king responding to Spencer's claims.

"While we do not comment on books as a matter of principle, His Majesty is mindful that the pain of fraternal grief can cloud reason, affect judgment and color memory in ways others do not recognize, even many years after such a loss," the statement said, according to People.

Princess Diana died in August 1997 at age 36 after suffering injuries sustained in a car accident in the Pont de l'Alma tunnel in Paris, France. Her partner, Dodi Fayed, and the driver, Henri Paul, also died in the crash.

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At the time of her death, she and King Charles had been divorced for one year. The two married in July 1981, announced their separation in December 1992 and finalized their divorce in August 1996.

Princess Diana died Aug. 31, 1997. She was 36. (Tim Graham Photo Library/Getty Images)

Prince Harry wrote about the decision to have him and his brother walk behind their mother's coffin in his 2023 memoir, "Spare," writing that "several adults were aghast" and that "mummy’s brother, Uncle Charles, raised hell," and that he told the royal family, "You can’t make these boys walk behind their mother’s coffin! It’s barbaric."

This is not the first time Spencer has spoken about his opposition to the boys walking behind their mother's coffin. He told BBC Radio 4’s Today in 2017 that he found the decision "bizarre and cruel."

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"It was the most horrifying half an hour of my life," he said. "It was just ghastly. And it was far worse than having to deliver a speech at the end of it. It was the worst part of the day by a considerable margin, walking behind, well, my sister’s body, with two boys who were obviously massively grieving their mother.

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"And it was this sort of bizarre circumstance where we were told you had to look straight ahead," he continued. "But the feeling, the sort of crashing tidal wave of grief coming at you as you went down this sort of tunnel of deep emotion, it was really harrowing actually, and I still have nightmares about it now."

Spencer found it "bizarre and cruel" to make Prince William and Prince Harry walk behind their mother's coffin at her funeral. (Jeff Overs/BBC News & Current Affairs via Getty Images)

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"Swan Song: Diana, My Sister" is set to be released Sept. 22.

Lori A. Bashian is an entertainment news writer at Fox News Digital.