Prince William’s feud with his uncle Andrew Mountbatten Windsor reportedly stems from a “rude” comment the former royal made about his nephew’s wife, Kate Middleton.

Author Andrew Lownie claimed in his royal biography, “The Rise and Fall of the Yorks,” that the Prince of Wales fully supported his father, King Charles III, in his decision to strip Andrew’s royal titles and honors due to his affiliation with late convicted sex offender Jeffrey Epstein.

“He [William] also loathes Sarah [Ferguson], Andrew’s ex-wife, and can’t wait for the day when his father throws them both out,” read an excerpt from a source noted in the book, per Daily Mail.

The real reason behind Prince William’s feud with his uncle Andrew Mountbatten Windsor has been revealed. Getty Images

The Prince of Wales reportedly took issue with Andrew after he made a “rude” comment about his nephew’s wife, Kate Middleton. WireImage

The book continues: “If Charles doesn’t, I guarantee you the first thing William does when he eventually becomes king is to get them evicted.”

The author also noted that William, 43, also got upset that Andrew, 65, and Ferguson, 66, attended Katharine, Duchess of Kent’s funeral in September.

“William, meanwhile, was ‘furious’ at being ambushed in this way, according to friends. He has been at pains to distance himself from his uncle and not be photographed with him,” Lownie claimed in the book.

“He believes his father has not dealt with him with sufficient firmness and that Andrew — and Sarah Ferguson — have done much to undermine the good work of other members of the Royal Family.”

Andrew’s exact comment remains unknown, but a royal biography claimed William was long in support of King Charles III’s decision to strip his brother of his royal titles. Getty Images

“He [William] also loathes Sarah [Ferguson], Andrew’s ex-wife, and can’t wait for the day when his father throws them both out,” read an excerpt from the “Rise and Fall of the Yorks.” Getty Images

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Andrew’s alleged remark toward Middleton, also 43, was not disclosed.

Reps for Kensington Palace did not immediately respond to Page Six’s request for comment.

Andrew was stripped of his royal titles by Charles on Thursday amid a renewed interest in the former prince’s ties to sex trafficker Epstein.

The title removal applies to Prince, Duke of York, Earl of Inverness, Baron Killyleagh and “His Royal Highness.” The honors affected include his Order of the Garter and Knight Grand Cross of the Royal Victorian Order.

The book also claimed William took issue with Andrew’s attendance at the Katharine, Duchess of Kent’s funeral in September. REUTERS

Andrew was stripped of his royal title and evicted from his royal lodging on Thursday amid a renewed interest in his ties to Jeffrey Epstein. WireImage

Page Six was told that Andrew will be relocating to a property on the private Sandringham estate after he was evicted from his royal lodging.

Ferguson, on the other hand, will make her own arrangements. She has also lost her Duchess of York title.

The former couple’s two daughters, Princess Beatrice, 37, and Princess Eugenie, 35, retained their royal titles amid the scandal.

At the time, we’re told Andrew “has not objected” to the effective changes.

We’re told Andrew will be relocating to a property on the private Sandringham estate. Getty Images

Andrew’s daughters, Princess Beatrice and Princess Eugenie, will retain their title. AP

Weeks prior to Charles’ official announcement, the former British Navy captain pledged that he would no longer use [his] royal title or honors because his ongoing allegations “distract from the work of His Majesty and the Royal family.”

Andrew wrote in a statement, “I have decided, as I always have, to put my duty to my family and country first. I stand by my decision five years ago to stand back from public life.”

However, Andrew added, “With His Majesty’s agreement, we feel I must now go a step further. I will therefore no longer use my title or the honours which have been conferred upon me.”

Andrew was previously sued by Virginia Giuffre for sexually assaulting her as part of Epstein’s sex trafficking ring. Getty Images

She and Andrew settled the case out of court the following year. (Andrew denied any wrongdoing.) Florida Southern District Court

Prior to her 2022 death, Queen Elizabeth II stripped Andrew of his military titles and patronages. AP

Back in 2021, Virginia Giuffre sued Andrew for sexual assault, claiming he allegedly raped her when she was a teenager as part of Epstein’s sex trafficking ring.

She and Andrew settled the case out of court the following year. [Andrew denied any wrongdoing.]

Before Queen Elizabeth II’s 2022 death, she stripped son Andrew’s military titles and patronages.