The revelation comes after Harry alleged he got into a physical fight with William in 2019 regarding Meghan.
While Buckingham Palace has officially remained mum on Prince Harry's bombshell memoir, Spare, sources close to Prince William say he's "privately seething and devastated" by what his brother has written in the upcoming autobiography.
Royal expert Katie Nicholl tells ET that William is also "incredibly angry at the level of detail" in which Harry has shared in his memoir. Harry recently claimed that William, or "Willy" as he refers to him in his book, physically assaulted him, leaving him visibly injured on his back after angrily speaking about Meghan Markle in 2019. He also referred to William as his "beloved brother and archnemesis" in the book in just one of the many bombshells.
That the brothers apparently went to blows is not all that surprising to Nicholl.
"Knowing how bad things have got between the brothers, it's not actually all that surprising that one of them just lost their temper and lashed out," she tells ET. "But to read about it in such detail is very painful, and one can only imagine very difficult for the Prince of Wales, Prince William, who of course isn't really able to answer back, isn't really able to share his side of the story."
Nicholl also tells ET that William is upset because his father, King Charles III, made numerous attempts to extend an olive branch to Harry and Meghan, but time and again, the couple has refused the good gestures. And Nicholl thinks she knows why they refused.
"The king invited Harry and Meghan to Sandringham for Christmas, they refused that invitation," Nicholl says. "The king has made a real point of extending an olive branch to the couple and I think knowing just how damaging this book was going to be for the royal family, Harry and Meghan made that decision to stay away over Christmas."
As for how the book's been received in the U.K., Nicholl says it sent shock waves across the pond.
"Jaws really are on the ground over here in the U.K.," Nicholl tells ET. "And, in terms of the royal family, we're not hearing anything officially. There's still no comment out of Kensington Palace, out of Buckingham Palace, but the repercussions of Harry's autobiography, the things that he's said are clearly being felt. My understanding is that the mood in the palace is one of shock, one of disbelief, and Prince William is said to be devastated and seething over this autobiography."
In a preview of an upcoming interview with ITV, the Duke of Sussex has expressed he wants both his father, Charles, and brother, William, "back" in his life amid ongoing tensions with the royal family.
"I would like to get my father back. I would like to get my brother back," Harry told ITV's Tom Bradby.
While Harry is willing to reconcile with the pair, he tells ITV that neither William nor Charles has shown any willingness to mend their strained relationship.
"They've shown absolutely no willingness to reconcile," Harry said.
And, as far as reconciliation goes, Nicholl tells ET she just doesn't see that happening.
"Prince Harry's given a number of interviews ahead of the release of his autobiography and he speaks about wanting to reconcile with his family," Nicholl says. "He talks about the door being open, but I think the feeling is that, actually, he's made sure that door has been firmly slammed shut."
She continued, "The idea of any reconciliation seems almost unthinkable, given what he has said, given the very personal nature of these attacks because that's what they are -- they are verbal attacks on members of the royal family. And not just any member of the royal family -- his father, his brother, his mother-in-law, his sister-in-law. No one is really spared in Spare."
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