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Princess Diana would have been "disappointed" in her sons’ strained relationship with each other after they once had an unshakable bond.

The claim is being made by her biographer Andrew Morton, who in 1992 wrote "Diana: Her True Story – In Her Own Words," a bombshell book that was based on tapes the Princess of Wales recorded and delivered to him secretly.

Most recently, the British biographer penned the book "Elizabeth & Margaret: The Intimate World of the Windsor Sisters," which explores their fierce devotion for each other, and the tensions they endured along the way.

"Obviously like any mother, Princess Diana would have been very disappointed and quite upset [over the alleged feud]," Morton told Fox News. "She’d be working towards a reconciliation."

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The Princess of Wales greets her sons Prince William and Prince Harry on the deck of the yacht Britannia in Toronto, when they joined their parents on an official visit to Canada, 23rd October 1991.

The Princess of Wales greets her sons Prince William and Prince Harry on the deck of the yacht Britannia in Toronto, when they joined their parents on an official visit to Canada, 23rd October 1991. (Photo by Jayne Fincher/Princess Diana Archive/Getty Images)

Rumors of a split between the brothers came to light in 2019 when British bestselling author Katie Nicholl claimed that after Prince Harry, 36, announced his engagement to Meghan Markle, 39, in 2017, Prince William, 39, grew increasingly worried for his younger sibling and his whirlwind romance with the American actress.

"William was quite concerned that the relationship had moved so quickly," Nicholl claimed in the TLC documentary "Kate V. Meghan: Princesses at War?". "And being close to Harry, you know, probably the only person close enough to say to Harry, ‘This seems to be moving quickly. Are you sure?’ And I think what was meant as well intended brotherly advice just riled Harry."

"Harry is hugely protective of Meghan," Nicholl continued. "He saw that as criticism, he interpreted that as his brother not really being behind this marriage, this union. And I don’t think things have been quite right ever since."

In a new book titled "Battle of Brothers: William and Harry – The Inside Story of a Family in Tumult," British historian Robert Lacey claimed that although relations between the princes improved by the time Harry and Markle tied the knot in May 2018, they later worsened when William learned that a senior palace aide made bullying allegations against Markle.

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It is believed that Prince Harry (left) and Prince William (right) have a strained relationship.

It is believed that Prince Harry (left) and Prince William (right) have a strained relationship. (Getty)

Lacey alleged William confronted Harry, but it led to a "fierce and bitter" argument. That confrontation led to William removing Harry and Markle from the joint office they shared.

William allegedly told a friend he felt "hurt" and "betrayed" as he’s always felt it was his duty to protect his younger brother. However, the British crown – and everything it stands for – came first.

"Harry, for his part, was equally furious that William should give credence to the accusations against Meghan, and he was fiercely combative in his wife’s defense," an excerpt published in the U.K. Times read.

Morton said that despite the alleged feud, Diana would have supported Harry’s move to California where he now resides with his wife and their two children. The Duke and Duchess of Sussex’s departures from royal duties began in 2020 over what they described as the British media’s intrusions and racist attitudes towards the former "Suits" star.

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Britain's Prince Harry poses with Meghan Markle in the Sunken Garden of Kensington Palace, London, Britain, November 27, 2017. REUTERS/Toby Melville - RC1DEF845A40

Prince Harry has since moved to California with his wife Meghan Markle and their two children. (Reuters)

"I think she would have applauded his move to the west coast of America," said Morton. "She was spending a lot of her time in America as well. Towards the end of her life, she was looking at houses to buy in Malibu. She felt California was a place of openness and possibility, not only for herself but for the boys."

Morton shared that Diana would have sat down with Markle and given her much-needed advice on navigating a royal life as an outsider.

"The thing that I’ve learned over the years is that it looks easy," Morton explained. "It’s not very difficult to become a member of the royal family. But it takes years to become comfortable within the role, especially as a woman in the public eye. William’s wife, Kate Middleton, had love and encouragement from a supportive household, but it also took a long time for her."

"Marrying into the royal family comes with rituals, traditions and rules," Morton shared. "You have to learn all this stuff before you can make a significant change. And even then, it will always come with challenges. Loyalty always belongs to the crown. That’s something not only Diana, but every woman in the House of Windsor had to learn as they carved out their own identity."

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Princess Diana had plenty of experience coping with media scrutiny and would have helped her daughter-in-law, American actress Meghan Markle, Andrew Morton claims.

Princess Diana had plenty of experience coping with media scrutiny and would have helped her daughter-in-law, American actress Meghan Markle, Andrew Morton claims. (Reuters)

Markle’s team has strongly refuted the bullying allegations when they were first reported by the U.K. Times in March shortly before the couple spoke to Oprah Winfrey in a televised interview.

"The Duchess is saddened by this latest attack on her character, particularly as someone who has been the target of bullying herself and is deeply committed to supporting those who have experienced pain and trauma," a spokesperson for the Duke and Duchess of Sussex said in a statement sent to Fox News.

"She is determined to continue her work building compassion around the world and will keep striving to set an example for doing what is right and doing what is good," the statement added.

All eyes will be on the brothers as they reunite on Thursday for the unveiling of a statue honoring Diana at Kensington Palace. July 1 would have been her 60th birthday.

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Britain's Prince William and Prince Harry, right, arrive at St Mark's Church in Englefield, England, ahead of the wedding of Pippa Middleton and James Matthews, Saturday, May 20, 2017. Middleton, the sister of Kate, Duchess of Cambridge is to marry hedge fund manager James Matthews in a ceremony Saturday where her niece and nephew Prince George and Princess Charlotte are in the wedding party, along with sister Kate and princes Harry and William. (Justin Tallis/Pool via AP)

Prince William and Prince Harry will reunite on July 1 to honor their mother Princess Diana. (The Associated Press)

Harry made the flight from California to his home country and has been isolating at Frogmore Cottage.

According to Kensington Palace, the brothers will be joined by close family members of the princess, as well as statute committee members, sculpture Ian Rank-Broadley and Pip Morrison, the designer of the Sunken Garden at the palace.

Due to COVID-19 restrictions, the guest list has been scaled down. Markle stayed behind with the couple’s two children: Archie Harrison, 2, and Lilibet Diana, who was born on June 4.

Plans were first announced in February 2017 and the statue was commissioned by Diana's sons to honor her life and legacy.

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Princess Diana, seen here with her boys, passed away in 1997 at age 36.

Princess Diana, seen here with her boys, passed away in 1997 at age 36. (Photo by Julian Parker/UK Press via Getty Images)

In March Harry acknowledged to Winfrey, 67, that he and his brother are on "different paths."

"I love William to bits," Harry explained. "He’s my brother. We’ve been through hell together. But we are on different paths."

While Harry described his relationship with William as "space," the prince stressed he "will always be there" for him.

Diana passed away in 1997 from injuries she sustained in a Paris car crash. She was 36.

The Associated Press contributed to this report.