Christopher Andersen, author of "Kate!," details why the Princess of Wales reportedly reached her breaking point in the feud between Prince William and Prince Harry.

Kate Middleton refused to stop trying to mend Prince William and Prince Harry’s strained relationship, but the Duke of Sussex’s remarks about King Charles’ mortality became the breaking point.

The claim was made by royal author Christopher Andersen, who spoke to numerous sources for his book "Kate!," which explores the Princess of Wales’ rise as one of the House of Windsor’s most influential figures.

"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.

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Author Christopher Andersen told Fox News Digital that Kate Middleton (left) had been determined to heal the rift between the princes — until Prince Harry’s 2025 BBC interview reportedly brought those efforts to a standstill. (Chris Jackson/POOL/AFP via Getty Images)

"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."

"Kate!: The Courage, Grace, and Power of the Woman Who Will Be Queen" by Christopher Andersen is available now. (Gallery Books)

"It was hurtful to Kate as well, I believe," said Andersen. "And, of course, it just ignited this firestorm of speculation about how sick the king really was at this point. And Kate pretty much felt that would be it. She wasn’t going to try anymore. So she has not. And as a result, there has been no chance of any … As a matter of fact, I don’t think we’re ever going to see them make amends. At least it’s not on the horizon now."

Catherine, Princess of Wales, attends a lunch at the rural Agriturismo "Al Vigneto" in Felino, Italy, on May 14, 2026, concluding a two-day visit to the region focused on early childhood development. (Phil Noble/Samir Hussein/WireImage)

Fox News Digital reached out to Kensington Palace for comment. A spokesperson for Buckingham Palace previously told Fox News Digital, "We don’t comment on such books."

In May 2025, Harry told the BBC he wanted to reconcile with his family after losing a court battle over his publicly funded security, which he said led to his father stopping talking to him.

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"I would love reconciliation with my family," said the 41-year-old. "There’s no point in continuing to fight anymore. I don’t know how much longer my father has."

The monarch, 77, is being treated for an undisclosed form of cancer.

According to Prince Harry in "Spare," King Charles pleaded with both Harry and Prince William after Prince Philip’s 2021 funeral, saying, "Please, boys, don’t make my final years a misery." (Karwai Tang/WireImage/Getty Images)

Andersen wrote that Kate, 44, was "disappointed" with Harry, while William, 43, was "apoplectic" with rage over the remarks, a courtier claimed. Andersen also wrote that Kate told William she "was done with Harry."

From left: Prince William, Kate Middleton and Prince Harry attend the Coach Core graduation ceremony for more than 150 apprentices at London Stadium on Oct. 18, 2017. Christopher Andersen wrote that after Harry's 2025 interview with the BBC, she "was done" with him. (Max Mumby/Indigo/Getty Images)

"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."

Andersen told Fox News Digital he wanted to explore Kate’s close friendship with Harry and what ultimately went wrong.

From left: Prince William, Kate Middleton and Prince Harry launched the Heads Together campaign to promote mental wellbeing at Kensington Palace in London on April 21, 2016. (Nicky J Sims/Getty Images for Royal Foundation)

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

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Prince Harry and Kate Middleton, seen here on April 25, 2019, had a close friendship before his royal exit. (Victoria Jones/WPA Pool/Getty Images)

"Of course, it all exploded after his marriage to Meghan Markle," Andersen claimed. "[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 Kate instantly clicked when they met. They shared "a screwball, somewhat slapstick, Monty Python-ish sense of humor — often at William’s expense."

From left: Kate Middleton, Prince William and Prince Harry participate in a race during a Heads Together team training day for the London Marathon at Olympic Park in London on Feb. 5, 2017. (Samir Hussein/WireImage)

"I liked making her laugh," Harry recalled in "Spare." "And I was quite good at it."

Prince Harry, seen here in 2016 with Prince William and Kate Middleton, was eager to meet his soulmate and turn their trio into a fabulous foursome, author Christopher Andersen claimed. (Jeff Spicer/Getty Images)

When Harry spoke about finding his soulmate, like William had, and joining the now-Prince and Princess of Wales as a foursome, she would reportedly encourage him by saying, "You’ll find someone. It just takes time."

In the summer of 2016, Harry began dating Meghan, an American actress, after being set up by a mutual friend. The relationship moved quickly, with the pair announcing their engagement in November 2017. They married in May 2018.

Prince Harry, Duke of Sussex, and Meghan, Duchess of Sussex, leave Windsor Castle after their wedding at St. George's Chapel on May 19, 2018, in Windsor, England. (James Devaney/GC Images)

Despite Kate and Harry’s incredibly close bond, Andersen said Kate had an awkward first meeting with the woman who would become her sister-in-law. He wrote that after saying, "It’s so wonderful to finally meet you," Meghan reportedly "wrapped her arms around Kate." The princess "seemed startled, then stiffened." Andersen noted that when Meghan met William months earlier, he "recoiled" when she went in for a hug.

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Meghan Markle and Kate Middleton attend the first annual Royal Foundation Forum in London on Feb. 28, 2018. Meghan married Prince Harry in May of that year. (Chris Jackson/Pool/AFP)

"I was a hugger," Meghan later said in the 2022 Netflix docuseries "Harry & Meghan." "Always been a hugger. I didn’t realize that was really jarring for a lot of Brits. I guess I started to understand very quickly that the formality on the outside carried through on the inside."

"What also surprised her was that William and Kate acted as if they had never heard of [her show] ‘Suits,’ even though Harry had told her they were die-hard fans of the series," Andersen wrote.

Meghan Markle starred as Rachel Zane in the legal drama "Suits." (Ben Mark Holzberg/USA Network/NBCU Photo Bank/NBCUniversal via Getty Images)

Andersen told Fox News Digital it was likely never destined for Kate and Meghan, 44, to become friends.

From left: Meghan, Duchess of Sussex, Prince Harry, Duke of Sussex, Prince William and Kate Middleton watch a flypast from the balcony of Buckingham Palace on July 10, 2018, in London to mark the centenary of the Royal Air Force. (Max Mumby/Indigo/Getty Images)

"Meghan is quintessentially Californian," he said. "The fact that she’s an actress and an influencer — these are things that are not merely that she’s an American and the sensibilities are different ... I don’t think Kate and Meghan were ever going to be great friends."

"Kate Middleton was willing to make tremendous sacrifices for her husband, frankly, for her country, because she realized that the monarchy is extremely important, not only to the U.K. but the Commonwealth," he continued.

Prince William, Prince of Wales, and Catherine, Princess of Wales, attend a service of thanksgiving at Westminster Abbey in London on the 80th anniversary of VE Day on May 8, 2025. (Jordan Pettitt/WPA Pool/Getty Images)

"And she wanted to do her job. She sees it that way. Loyalty is a big thing for her. Again, she believes in king and country to some extent, not at the expense of her family, but she was willing to do what comes with being the Princess of Wales and the future queen."

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Prince Harry and Meghan, Duchess of Sussex, arrive at the Endeavour Fund Awards at Mansion House in London on March 5, 2020. Soon after, the couple moved to California. (Justin Tallis/AFP)

Harry and Meghan, the Duke and Duchess of Sussex, stepped back as senior royals in 2020, citing intrusive British press coverage and a lack of support from the palace. After moving to California that year, they aired their grievances in interviews and documentaries. Harry’s 2023 memoir further strained his relationship with the royals.

Andersen said Kate "made many, many attempts" to bring the brothers back together. While there appeared to be hope for reconciliation at Prince Philip’s funeral in 2021, the relationship quickly soured again. In "Spare," Harry described a heated exchange with his brother that he compared to a "duel."

Prince Harry’s memoir "Spare" was published on Jan. 10, 2023. (Lev Radin/Pacific Press/LightRocket)

"I took it all in: his familiar scowl, which had always been his default in dealings with me; his alarming baldness, more advanced than my own; his famous resemblance to Mummy, which was fading with time," Harry wrote. "With age. In some ways, he was my mirror, in some ways, he was my opposite. My beloved brother, my arch nemesis, how had that happened?"

Prince Harry and Prince William, seen here in 2021 to honor their late mother's 60th birthday, are said to not be on speaking terms. (Dominic Lipinski/Pool/AFP)

In "Spare," Harry also claimed that in one incident, William called his wife "difficult," "rude" and "abrasive."

"He grabbed me by the collar, ripping my necklace, and he knocked me to the floor. I landed on the dog's bowl, which cracked under my back, the pieces cutting into me. I lay there for a moment, dazed, then got to my feet and told him to get out."

Prince William, Prince Harry and Peter Phillips walk behind Prince Philip's coffin carried by a Land Rover hearse during the funeral procession at Windsor Castle in Windsor, England, on April 17, 2021. (Mark Large-WPA Pool/Getty Images)

Kensington Palace and Buckingham Palace declined to comment on the claims in the book.

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Author Christopher Andersen claimed that Kate Middleton and Meghan Markle were never likely to become close friends. (Richard Pohle/WPA Pool/Getty Images)

While Harry and Kate were exceptionally close, Andersen wondered whether Meghan’s outlook on loyalty to the crown as a newcomer complicated that dynamic.

"I think Meghan is very charming," said Andersen. "She’s very intelligent, and I think she wanted the best for Harry. But I also think maybe [Kate] saw quite quickly that Meghan prioritized things that the rest of the royals wouldn’t. Maybe Meghan didn’t grasp the weight of the responsibility that was being thrust upon her.

Since marrying in 2018 and making their royal exit in 2020, the Duke and Duchess of Sussex have been attempting to carve out their own independent roles outside the monarchy. (Joshua Sammer/Getty Images)

"[Kate] can see when someone else isn’t really willing to step up and make that kind of sacrifice, and maybe prioritizing the country, this thousand-year-old institution, one of the greatest in the world, over her own life. That’s what Kate has done, really. And Meghan, from the start, made it clear that she wasn’t willing to do that."

Stephanie Nolasco covers entertainment at Foxnews.com.

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