As King Charles III celebrates his 75th birthday on Tuesday, a guest will be noticeably missing from the festivities: his younger son.

A spokesperson for Prince Harry shot down a report from the U.K.’s Sunday Times stating that the Duke of Sussex is "not making the trip from California," where he settled in 2020 with his wife, Meghan Markle. The rep told The Messenger that "there has been no contact regarding an invitation to His Majesty’s upcoming birthday."

Christopher Andersen, author of "The King," told Fox News Digital he wasn’t surprised by the revelation. He noted that the damage was already done when the 39-year-old prince wrote about the king's beloved wife, Camilla, in an explosive memoir titled "Spare."

KING CHARLES' 75TH BIRTHDAY: ROYALS 'RELIEVED' PRINCE HARRY, MEGHAN MARKLE NOT ATTENDING CELEBRATION: EXPERT

Prince Harrys book Spare on display at a store

Prince Harry's bombshell book was published in January 2023. (Lev Radin/Pacific Press/LightRocket)

"My feeling is that Charles can so easily turn his back on Harry because Harry committed the one unforgivable sin: Harry dissed Camilla in his book, ‘Spare,’ branding her a ‘villain,’" Andersen said.

"He could have trashed [Prince] William and the monarchy all he wanted, but King Charles is wildly, blindly devoted to Queen Camilla," Andersen said. "King Charles III angers easily, and he holds grudges."

Harry’s memoir, which was published in January of this year, details his life behind palace walls before he made his royal exit. In it, the father of two accused his stepmother of leaking private conversations to burnish her own reputation.

Queen Camilla wearing a green dress and a beige hat next to Prince Harry in uniform

In interviews to promote "Spare," Prince Harry singled out Queen Camilla's efforts to rehabilitate her image with the British people. (Max Mumby/Indigo/Getty Images)

In interviews to promote the book, Harry accused members of the royal family of "getting into bed with the devil" to gain favorable tabloid coverage. He singled out Camilla’s efforts to rehabilitate her image with the British people after her affair with his father.

"That made her dangerous because of the connections that she was forging with the British press," Harry told CBS. "There was open willingness on both sides to trade information. And with a family built on hierarchy, and with her on the way to becoming queen consort, there was going to be people or bodies left in the street."

In the book, Harry also said that he and his older brother, William, "begged" their father not to marry Camilla for fear she would become a "wicked stepmother."

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A close-up of King Charles and Queen Camilla looking at each other as they wear their robes and crowns

King Charles III and Queen Camilla were crowned on May 6, 2023, in London. (P van Katwijk/Getty Images)

The allegations about Camilla are particularly sensitive because of her role in the acrimonious breakdown of Charles’ marriage to the late Princess Diana, who as William and Harry’s mother.

Diana once described Camilla as the third person in her marriage to Charles. While many members of the public initially shunned Camilla, she has won fans by taking on a wide range of charitable activities and has been credited with helping Charles appear less stuffy and more in tune with modern Britain.

Writing about his father’s 2005 wedding to Camilla, Harry said, "I had complex feelings about gaining a stepparent who, I believed, had recently sacrificed me on her personal PR altar." Still, he says he wanted his father to be happy.

"In a funny way, I even wanted Camilla to be happy," he wrote. "Maybe she’d be less dangerous if she was happy?"

Queen Camilla smiing next to King Charles as they both wear black

King Charles III and Queen Camilla attend the Royal British Legion Festival of Remembrance ceremony in London on Nov. 11, 2023, as part of Remembrance Day, which commemorates the end of World War I. (Chris Jackson/Pool/AFP)

Camilla, 76, was crowned queen alongside her husband in May this year.

"The fact that Harry and Meghan weren't even invited to the king's milestone 75th birthday really speaks volumes about just how irreconcilable the Sussexes and the Windsors are," said Andersen. "You can only burn bridges so many times."

"The bottom line is that the Sussexes and the rest of the royal family will continue to have nothing to do with each other for the foreseeable future, full stop," Andersen said.

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Prince Harry and Meghan Markle smiling and applauding in the middle of a crowd

The Duke and Duchess of Sussex stepped back as senior royals in 2020. They live in California with their son, Prince Archie, and daughter Princess Lilibet. (Joshua Sammer/Getty Images)

British royals expert Hilary Fordwich told Fox News Digital that the royals are expected to keep calm and carry on for the king's big day. She noted that the family will have a "low-key celebration following his wishes, given all the festivities around his recent coronation."

"He’ll be spending his special day just with friends and family at his London home where he’s lived with Queen Camilla for two decades, Clarence House," Fordwich explained.

"This intimate affair is restricted to close friends and family," Andersen added. "This is the real birthday bash, usually attended by scores of well-heeled guests, including lots of folks with titles as well as a smattering of celebrities and deep-pocketed contributors to Charles's favorite charities."

King Charles smiling to a group of people holding a cup and saucer

King Charles meets guests during a 75th birthday party that was hosted for him by the Prince's Foundation at Highgrove House in Tetbury, England, on Nov. 13, 2023. (Chris Jackson/Pool/AFP)

But charitable activities will be "the focus" this year," said Fordwich.

"His milestone birthday will also be marked by his visit to a surplus food distribution center outside of London where he will launch the Coronation Food Project, spearheaded by Baroness Louise Casey and Dame Martina Milburn," she said. "It’s an ambitious initiative aimed at reducing the amount of food waste by distributing food otherwise destined for landfills."

The celebrations started on Monday when Charles and Camilla hosted a tea dance with a group of 75-year-olds at the king’s Highgrove estate, Fordwich said. It was followed by a private dinner at which the king hosted nurses and midwives at Buckingham Palace in London to salute the National Health Service's 75th birthday.

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King Charles in a suit cutting his birthday cake

King Charles is expected to have a more low-key affair for his birthday with close family and friends. (Chris Jackson/Pool/AFP)

"The shadow regarding the king’s celebrations is the coverage commenting on the absence of his estranged younger son," said Fordwich.

In an interview with The Telegraph to promote "Spare," Harry told the royal family to "come clean," noting he wanted to share the "truthful" side of his story and defend his wife.

"You know what you did, and I now know why you did it," he said in January, claiming that his family was treating him as if he were "delusional and paranoid." He added that "if people had listened" to him sooner, the rift wouldn’t have deepened so badly.

Prince Harry looking somber next to King Charles and Queen Camilla

Prince Harry, center, is seen attending Queen Elizabeth II's funeral on Sept. 19, 2022. King Charles III ascended to the throne upon his mother's death. (Isabel Infantes/Pool/AFP via Getty Images)

"Sadly, not only is Prince Harry disaffected from his own family, but his children are, too," Fordwich claimed. "They don’t know their cousins and are likely not to ever have a relationship with them."

"Also, a sad sight was Prince Harry on Armistice Day, not with his family laying a wreath as he had done with all his royal family in years past, but in the U.S. wearing his poppy where they aren’t recognized," Fordwich continued. "Needless to say, [it's] a sign that he’s still hanging on to his British roots even after ripping them up himself."

True Royalty TV co-founder Nick Bullen previously told Fox News Digital that Camilla was allegedly outraged by Harry’s remarks in "Spare."

QUEEN CAMILLA 'FURIOUS' WITH PRINCE HARRY'S 'SPARE,' WON'T FORGIVE 'BOMB-LIKE' EXPOSE: INSIDER

Prince Harry, Duke of Sussex, is seen at King Charles IIIs coronation

Prince Harry swiftly headed back to California after his father's coronation in May. (Ben Stansall)

"I know from various people who are close to her [that] she was pretty furious with what was being said about her," he said. "But she is not someone that is going to be phoning Harry or Meghan and shouting down on the phone at them."

"She just moves on," Bullen said. "I don’t think she’s necessarily going to forgive, and I don’t think she’s necessarily going to forget. But it is not something that she dwells on, I’m told. She plays a much longer game, and she plays with a much straighter bat. She was, I’m told, annoyed by what was said but moved on pretty quickly from what was said."

Bullen is an award-winning documentarian who has been producing programs about the British royal family for 20 years. He has also worked closely with the king for about a decade.

Prince Harry kissing his father on the cheek

Prince Harry is seen here with his father in 2018. The Duke of Sussex made his royal exit two years later. (Matt Dunham/WPA Pool/Getty Images)

"The book was full of bomb-like moments that were going off in the palaces around Britain," said Bullen. "For the king and [Prince William], the comments that Harry made about their wives ... I think were the most hurtful for the royal family. You can fight with your brother, you can fight with your dad, but really should you be throwing brickbats at your sister-in-law and stepmother? And particularly when they’re not going to be able to respond? So, I think those were the ones that hurt the most."

The Associated Press contributed to this report.