Prince Harry's close bond with Kate Middleton may have made Meghan Markle "incredibly envious and then jealous," a royal expert has suggested.

While discussing Harry's recent one-day trip to the U.K., which did not include a meeting with Prince William and Middleton, Ingrid Seward, the editor of Majesty magazine, touched on the Duke of Sussex's sibling-like relationship with his sister-in-law.

The royal author told The Telegraph that the Prince of Wales allegedly "cannot forgive" his younger brother for making several allegations about Middleton in his memoir "Spare," including claims that she made Markle cry and "grimaced" when the Duchess of Sussex asked to borrow her lip gloss.

"Unfortunately, it points to Meghan, doesn't it? Maybe Harry was a little in love with Kate," Seward suggested to the outlet, before explaining, "I don't mean physically but mentally. Remember when it was just the three of them?"

Prince William, Kate Middleton and Prince Harry
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Prince Harry, Kate Middleton and Prince William laugh as they host a tea party on the grounds of Buckingham Palace to honour the children of those who have died serving in the armed forces on May 13, 2017 in London, England.

According to Seward, Prince Harry had expressed a longing for a sister and "adored" Middleton like a sibling after she entered his older brother's life.

"He was always there, at Kensington Palace, in their fridge, you know, 'What's for supper?'" Seward claimed of Prince Harry's past dynamic with Prince William and Princess Kate.

"I think that Meghan must have been incredibly envious and then jealous of Kate," the royal author claimed. "I heard Meghan actually thought she was going to be a princess and live in Windsor Castle. Instead, there's William and Kate with this beautiful house, while they are stuck in Nottingham Cottage which Harry used to call 'my hovel.'"

Kate Middleton and Meghan Markle
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Kate Middleton and Meghan Markle attend day twelve of the Wimbledon Lawn Tennis Championships at All England Lawn Tennis and Croquet Club on July 14, 2018 in London, England.

The royal author's remarks come against the backdrop of Prince Harry's recent return to the U.K. to visit his father, King Charles III, following the announcement of the monarch's cancer diagnosis.

However, the short trip did not include a visit to his older brother and sister-in-law, the latter of whom recently underwent abdominal surgery.

The "Royal Trio," as Prince William, Princess Kate, and Prince Harry were once known, was previously inseparable.

However, Prince Harry reportedly became estranged from the Prince and Princess of Wales after he and Markle stepped back from their roles as working royals and moved to California in 2020.

Prince William, Prince Harry and Kate Middleton
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Prince William, Prince Harry and Kate Middleton smile as they take part in 'welly wanging', with children and representatives from charities and Aardman Animations, during a meeting of the Charities Forum at BAFTA on October 26, 2015 in London, United Kingdom.

Tensions were further fueled by the release of Prince Harry's book and the Sussexes' Netflix series, in which Harry claimed that men in the royal family often face a "temptation or an urge to marry someone who would fit the mold, as opposed to somebody who [they] are perhaps destined to be with."

Robert Hardman suggested in his biography "The Making of a King: King Charles III and the Modern Monarchy" that Prince Harry and Prince William's rift widened after what the latter viewed as a "blatant attack" on Middleton, Us Weekly reported.

An unnamed source previously told the outlet that Middleton was "appalled" that Prince Harry dragged "her name through the dirt" in his docuseries and book.

Prince Harry and Kate Middleton
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Prince Harry and Kate Middleton leave the church after the Royal wedding of Zara Phillips and Mike Tindall at Canongate Kirk on July 30, 2011 in Edinburgh, Scotland.

Prince Harry and Markle have not made any public comments on Princess Kate's health situation since Kensington Palace announced last month that the Princess of Wales was hospitalized after undergoing a "planned abdominal surgery" on Jan. 16.

But an unnamed insider told the U.K.'s Mirror last month that the Sussexes privately sent well-wishes to Middleton and King Charles, who at the time was preparing to undergo a procedure for benign prostate enlargement.

On Feb. 5, Buckingham Palace said in a statement that King Charles was diagnosed with "a form of cancer" following the procedure. No other details were provided.