Prince Harry claims that King Charles joked "who knows if I'm even your real father?" during rumours that the Duke of Sussex was the son of Princess Diana and her ex-lover Major James Hewitt.
In his new tell-all book, Spare, Prince Harry delves into his past while discussing his mother Diana's affair with Major Hewitt.
The book, which is not due out in the UK until January 10, was accidentally released today in Spain.
The Duke of Sussex reportedly mentions in the memoir that his dad was fond of anecdotes and that one of his stories included a visit to a mental asylum where he met a man claiming to be the Prince of Wales - a role that the King had at the time of the visit.
According to Prince Harry's recount of the story, Charles wagged his finger at the patient and said he was the only Prince of Wales.
Harry adds that his dad had a tendency for ending his stories with a "philosophical flash" and in this instance Charles mused about whether anyone could be sure they were not the mentally ill one themselves, "living in a fantasy".
Prince Harry then claims in the book that Charles joked: "Who knows if I'm even your real father? Perhaps your father really is in Broadmoor, my dear son!"
The Prince writes in the memoir that the joke was "in poor taste" given the rumour going around at the time that his real father was James Hewitt following his mother's affair, reports the Daily Mail.
Princess Diana shared a five-year affair between 1986 and 1991 with Mr Hewitt when he was a Household Cavalry officer.
The relationship was confirmed by the Princess of Wales in the now-infamous Panorama interview with journalist Martin Bashir.
The rumour mill has often claimed that Prince Harry shares his looks with Mr Hewitt, leading some people to think that he was a product of the affair.
Writing in his memoir, Harry makes reference to the rumours.
He claims that the joke was funny in light of the rumours about who his real father is and said that one cause of the gossip was his ginger hair.
The Duke then writes that readers of tabloid publications lapped up the idea that he may not be Prince Charles' son.
Harry goes on to ponder whether it was to help paint him as a joke.
He says it didn't seem to matter that Diana hadn't even met Major Hewitt when Harry was born.
Harry also claims some journalists went so far as to find his DNA to verify the rumours.
Other bombshells from the new book include Harry's account of an alleged physical attack on him by brother William during a row over Meghan.
Meanwhile, Harry reportedly dedicates his memoir to his wife, children and late mother - but makes no mention of his father or his brother.
Kensington Palace and Buckingham Palace have declined to comment on the leaked claims from Harry’s book.
