Prince Harry and Prince William have been in contact to prepare for their late mother’s birthday, but the relationship still has a long way to go.

"There have been conversations after Lili was born and to make plans for next week, but things are still very strained," a mutual friend of the brothers told Vanity Fair royal correspondent Katie Nicholl on Tuesday.

An insider told Nicholl that Harry, 36, is returning to the U.K. alone ahead of the statue unveiling honoring the brothers’ late mother Princess Diana. The event will take place on July 1 on what would have been her 60th birthday.

According to the outlet, Harry’s grandmother Queen Elizabeth II is "desperate" to see her 2-year-old grandson Archie. However, the toddler will be staying at home in California with his mother Meghan Markle and his baby sister Lilibet "Lili" Diana, who was born on June 4.

PRINCE WILLIAM, PRINCE HARRY HAD A ‘FIERCE AND BITTER’ ARGUMENT OVER MEGHAN MARKLE AFTER BULLYING CLAIMS: BOOK

Vanity Fair's royal correspondent Katie Nicholl reported that Prince Harry is traveling this week back to the U.K. (Kevin Mazur/Getty Images for Global Citizen VAX LIVE)

"Meghan will stay at home with the children," a pal told Nicholl. "Harry is flying alone."

According to the friend, the duchess is allegedly "concerned" about Harry’s solo trip and the reception he’ll receive back home. Nicholl shared that the duke was "privately shocked" by the "frosty welcome" he received from some family members when he returned to England in April for his grandfather Prince Philip’s funeral. 

Several family members "were still reeling" from the couple’s bombshell interview with Oprah Winfrey in March.

According to the outlet, the brothers "have agreed to a truce" to avoid media spectacle surrounding their relationship. It is believed they will give separate speeches.

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A pal of the brothers claimed to Katie Nicholl that their relationship is still very strained. (Getty)

"They know the eyes of the world will be on them – watching their every move – so what you will see is a carefully choreographed show of unity for that day, at least," a source claimed.

The outlet reported that Harry will likely stay at Frogmore Cottage, the U.K. base for the Duke and Duchess of Sussex. He will be close to the queen and his cousin Princess Eugenie. It’s unknown how long he will stay in the U.K.

Royal author Ingrid Seward claimed the queen, 95, invited her grandson for lunch, but it is unlikely she will discuss current matters in great detail "because she does not like confrontation." However, the meeting "will be an opportunity for them to have a private and open discussion."

Nicholl’s report came shortly after new details emerged about William and Harry’s alleged fallout.

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British historian and writer Robert Lacey pictured at the Edinburgh International Book Festival where he talked about his popular books about the British monarchy.  (Photo by Colin McPherson/Corbis via Getty Images)

Royal biographer Robert Lacey, who is also the official consultant on Netflix’s "The Crown," has updated his book "Battle of Brothers: William and Harry – The Inside Story of a Family in Tumult" with added chapters and new material. In the original book, which was published in 2020, the British historian addressed the ongoing woes impacted by the royal family.

In new excerpts published by the U.K. Times on Saturday, the veteran writer further explored the alleged feud between the brothers.

Rumors of a split came to light in 2019 when Nicholl claimed that after Harry announced his engagement to Markle, 39, in 2017, William, 39, grew increasingly worried for his brother 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."

VERDICT FROM MEGHAN MARKLE’S BULLYING INVESTIGATION COULD BE DELAYED TO 2022: REPORT

Prince Harry, Duke of Sussex, Meghan, Duchess of Sussex, Prince William, Duke of Cambridge, Catherine, Duchess of Cambridge and Prince Charles, Prince of Wales attend the Commonwealth Day Service 2020 on March 9, 2020, in London, England.  (Photo by Phil Harris - WPA Pool/Getty Images)

"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."

Now, Lacey wrote that although relations between the princes improved by the time Harry and Markle tied the knot in May 2018, they worsened in October 2018 when William, 39, learned that a senior palace aide made bullying allegations against Markle.

According to Lacey, William confronted Harry after he learned of the allegations. However, it led to a "fierce and bitter" argument. That confrontation led to William removing Harry and Markle from the joint office they shared.

"William threw Harry out," a friend alleged, as quoted by the outlet.

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Prince William (right) is second in line to the British throne. (Getty Images      )

Lacey alleged that the angry confrontation first occurred over the phone and then in person when William asked Harry about the bullying claims.

"When Harry flared up in furious defense of his wife, the elder brother persisted," read the excerpt. "Harry shut off his phone angrily, so William went to speak to him personally. The prince was horrified by what he had just been told about Meghan’s alleged behavior, and he wanted to hear what Harry had to say. The showdown between the brothers was fierce and bitter."

William allegedly told a friend he felt "hurt" and "betrayed" as 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," the excerpt read.

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Meghan Markle

Meghan Markle gave birth to a baby girl named Lilibet "Lili" Diana on June 4. (Photo by Getty       )

A rep for the Duke and Duchess of Cambridge, as well as the Duke and Duchess of Sussex, didn’t immediately respond to Fox News’ request for comment. However, Markle’s team strongly refuted the bullying allegations when they were first reported by the Times in March shortly before the couple spoke to Winfrey in a televised interview.

Lacey previously told Fox News it will take a while for the relationship between the brothers to heal.

"I think it would always have happened that Meghan and Harry would live on the other side of the world," he explained. "I think Meghan and Harry will remain based in North America. Unless the brothers can find a way to reconcile, they will have to go their separate ways and keep mutual respect for each other."

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. The family now resides in the coastal city of Montecito, Calif.

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This image provided by Harpo Productions shows Prince Harry, from left, and Meghan, The Duchess of Sussex, in conversation with Oprah Winfrey. (AP)

In the wake of quitting royal duties, they gave an explosive TV interview to Winfrey, in which the couple described painful comments about how dark their son Archie’s skin might be before his birth. The duchess talked about the intense isolation she felt inside the royal family that led her to contemplate suicide.

Buckingham Palace said the allegations of racism made by the couple were "concerning" and would be addressed privately.

The Associated Press contributed to this report.