A royal expert said the Duke of Sussex's choice not to attend a memorial party for his mother Diana, Princess of Wales, is likely to put a bigger gap between him and his brother. Prince Harry will not be among those attending a celebration at Kensington Palace on Tuesday night to commemorate his mother's 60th birthday.

The event, which had been postponed because of the continuing COVID-19 pandemic, was meant to honor benefactors for their contributions to the Princess Diana commemorative statue in the palace grounds.

In July, Prince Harry traveled alone to the unveiling of the statue, leaving his wife Meghan Markle and children Archie and Lilibet at home in California.

Royal expert says Prince William to include Prince Harry to his speech

"I think it's extremely unfortunate that Prince Harry didn't go, and I think he'd have been sadly missed," royal expert Duncan Larcombe, author of 'Prince Harry: The Inside Story,' told OK!.

Duncan feels that when it comes to thanking contributors during the party, Prince William will do the proper thing and include Prince Harry. With Prince Harry's declaration that he's working on a memoir and the royal family's recollections of the Oprah interview still fresh in their minds, it's no wonder that the brothers' relationship is strained, according to Duncan.

Despite the animosity, Duncan sees Harry's absence at the funeral party as a "missed opportunity." After remaining in California, Prince Harry will skip a ceremony at Kensington Palace tonight to honor his mother.

The British press speculated at one point that the Duke of Sussex -- and maybe Meghan Markle as well-- might fly over to Britain for the occasion to thank financial sponsors of a statue dedicated to his mother.

However, it was announced earlier this month that Prince Harry will not be attending the trip, leaving Prince William to host the event on his own, Newsweek reported. The princess' friends, relatives, and former employees will meet at the castle tonight for a party.

In July, on what would have been Diana's 60th birthday, the brothers unveiled the monument jointly in Kensington Palace's Sunken Garden. It was just their second face-to-face meeting since Harry and Meghan left their royal duties for a new life in America in April, after Prince Harry's attendance at Prince Philip's funeral.

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When will Prince Harry return to the UK?

It is yet unknown whether Prince Harry will return to the United Kingdom this year. The next important occasion on the royal calendar is Remembrance Sunday, November 14, when royals will assemble at the Cenotaph in London, Britain's national war memorial.

On November 10, Prince Harry will present an award at the Intrepid Museum's 2021 Salute to Freedom Gala in New York. Travelers from the United States, on the other hand, are no longer obliged to quarantine upon arrival in the United Kingdom, thus Prince Harry could possibly go to the United Kingdom after the event.

Last year, the royals were embroiled in a feud over a request for a wreath to be put on Prince Harry's behalf while he was away on Remembrance Sunday. Meanwhile, Andrew Morton believes Prince William had a role in Prince Harry and Meghan Markle's decision to leave the royal family.

'Meghan: A Hollywood Princess,' a book on the former 'Suits' star's rise to prominence and whirlwind marriage to Prince Harry, was published in 2018 by Princess Diana's biographer. The British author's book, titled 'Meghan and the Unmasking of the Monarchy,' was re-released on Tuesday.

It covers the Duke and Duchess of Sussex's life behind palace doors, as well as what eventually led to their choice to stand aside as senior royals, with fresh chapters and interviews with some of Markle's family members and closest friends.

Morton stated that William, the second in line to the British throne, had a role in their decision to back down. Morton said Prince William did not immediately embrace his sister-in-law because he was concerned about Markle's Hollywood history and how it might affect the palace, as per Fox News.

Related Article: Prince Harry, Meghan Markle Will Not Attend Tribute For Princess Diana; Do The Sussexes Plan To Christen Lilibet in The UK?