Princess Catherine reached her "final straw" with Prince Harry following his remarks about King Charles' cancer diagnosis, according to a royal author.

In his newly released biography of the 44 year old Princess of Wales, Kate! The Courage, Grace, and Power of the Woman Who Will Be Queen, royal biographer Christopher Andersen claims that Catherine had initially refused to abandon her efforts to help repair the fractured relationship between her husband, Prince William, and Harry.

The brothers have been at odds ever since Harry and his wife, Meghan Markle, officially stepped back from their roles as senior royals and relocated to the U.S.

Yet the 41 year old Duke of Sussex's comments regarding King Charles' mortality during an early May BBC interview brought Catherine's reconciliation efforts to an abrupt halt.

In his new book, which charts the future Queen's emergence as one of the Royal Family's most prominent figures, Andersen claims Catherine voiced her concerns to William regarding Harry's remarks.

"She would not give up," Andersen told Fox News Digital. "Even after Harry threw William under the bus in his memoir Spare, she was willing to keep trying. And the final break came."

"The final straw came really shortly after the king was diagnosed with cancer, when Harry [essentially] said, 'I'd like to mend fences because we just don't know how long [my father] has to live.'

"Well, that kind of statement can be very hurtful to somebody who doesn't really want to consider their own mortality and is battling cancer." During his BBC interview, Harry spoke of how his legal battle over his security arrangements had strained relations with his family, yet maintained that he is eager to make amends with his father.

"I would love reconciliation with my family. There's no point in continuing to fight anymore. Life is precious. I don't know how much longer my father has," he said about Charles, 77, who was diagnosed with an undisclosed form of cancer in 2024. "He won't speak to me because of this security stuff, but it would be nice to reconcile."

At the time, Harry also insisted that his father should be receptive to reconciliation and assist in resolving his security concerns.

"There is a lot of control and ability in my father's hands," he said. "Ultimately, this whole thing could be resolved through him. Not necessarily by intervening, but by stepping aside, allowing the experts do what is necessary."

In March 2024, the Princess of Wales announced her cancer diagnosis, following major abdominal surgery.

Catherine completed chemotherapy in September 2024 and gradually resumed her royal duties over the subsequent months. In January 2025, she confirmed that she is in remission.

Anderson told Fox News Digital that Harry's comments regarding Charles' cancer were "hurtful" to Catherine and "ignited this firestorm of speculation about how sick the king really was at this point." The author claimed that Catherine subsequently decided she would no longer attempt to involve herself in Harry and William's relationship. Anderson doesn't think there's any chance the brothers will make amends.

Fox News Digital reached out to Kensington Palace for comment. However, a spokesperson for Buckingham Palace previously told the outlet, "We don't comment on such books." The Express US has also reached out to Buckingham Palace for comment.

In his book, Andersen wrote that Catherine was "disappointed" with Harry, while William was "apoplectic" with rage over the remarks, according to a courtier. Andersen further wrote that Catherine informed William she "was done with Harry."

"Princess Catherine is the sweetest, most loving person you could ever know," a Sandringham staffer told Andersen. "But like everyone else, she has her limits.

"What interests me most was the relationship between Catherine and the two brothers and the fact that she acted as a kind of go-between," Andersen explained to the outlet.

"William, Harry and Kate had a very close relationship. [Harry] referred to Kate as the sister he never had. They made each other laugh. It was a very warm and loving relationship for a long time.

"Of course, it all exploded after his marriage to Meghan Markle.

"[But] ever since that point, she was the one person in the royal family who wouldn't give up on them." Andersen wrote that Harry and Catherine instantly bonded when they met, sharing "a screwball, somewhat slapstick, Monty Python-ish sense of humor - often at William's expense."

In his bombshell memoir, Harry wrote of Catherine, "I liked making her laugh. And I was quite good at it." Catherine would later reportedly urge him to seek a partner, telling him: "You'll find someone. It just takes time."

However, Andersen revealed that Catherine experienced an uncomfortable first encounter with Meghan. He wrote that after saying, "It's so wonderful to finally meet you," Meghan reportedly "wrapped her arms around Kate."

Catherine "seemed startled, then stiffened," and Andersen observed that when Meghan had met William several months prior, he "recoiled" when she moved in for a hug, despite her being a self-proclaimed "hugger."

Andersen suggested it was highly unlikely that Catherine and Meghan, 44, would ever forge a friendship. Following Harry and Meghan's departure from the Royal Family, Catherine "made many, many attempts" to reconcile the brothers, yet nothing appears to have mended their fractured relationship.