Harry states in his memoir that he and William "begged" their father not to marry Queen Consort Camilla, with whom Charles was having an affair while married to their mother.
Prince Harry's memoir "Spare" officially goes on sale next week but much of its contents have already been leaked in British and US media.
Here are some of the notable details.
Harry claims his older brother Prince William
attacked him during an argument about his wife Meghan.
"He grabbed me by the collar, ripping my necklace, and he knocked me to the floor," he writes in the book, according to The Guardian.
"I landed on the dog's bowl, which cracked under my back, the pieces cutting into me. I lay there for a moment, dazed, then got to my feet and told him to get out."
Harry refers to William as his "beloved brother and arch-nemesis".
"There has always been this competition between us, weirdly," he explained in an interview with the US television show "Good Morning America" set to air next Monday.
"I think it really plays into, or is played by, the 'heir/spare' (issue)," he added, referring to his traditionally diminished royal role compared to William, who is first in line to the throne.
The brothers' father, King Charles III, pleaded with his sons to stop fighting at a meeting after the funeral of his father, Prince Philip, in April 2021.
"Please, boys. Don't make my final years a misery," he told them, according to the memoir.
Harry also describes the "secret meeting" as resembling a duel rather than reconciliation.
William claimed he did not know why Harry had exiled himself in the United States, Harry writes, while his father "was looking at me with an expression that said, 'me neither'".
Harry reveals that Charles used to joke about whether he was really his father.
"Who knows if I'm really the Prince of Wales? Who knows if I'm even your real father?" Harry writes, according to The New York Post.
"He'd laugh and laugh, though it was a remarkably unfunny joke, given the rumour circulating just then that my actual father was one of Mummy's former lovers: Major James Hewitt."
Harry's mother, Princess Diana, had a five-year affair with Hewitt, an ex-cavalry officer, which gave rise to persistent speculation that he could be Harry's father.
Noting that Hewitt has similar "flaming ginger hair", Harry lambasts the claim as driven by tabloid "sadism".
"My mother didn't meet Major Hewitt until long after I was born," he notes.
'Heir and spare'
Harry writes in the book about his father's delight when Diana gave birth to a so-called "spare" in 1984.
Charles supposedly told his wife that Harry's arrival meant she had now given him both an heir and a spare -- and that his work was done.
Harry states that he and William "begged" their father not to marry Queen Consort Camilla, with whom Charles was having an affair while married to their mother.
He recounts that the brothers said they would not stand in the way of Charles' relationship with her, but asked that they did not marry.
Harry also details how he felt when meeting Camilla for the first time, likening it to having an "injection".
"I remember wondering... if she would be cruel to me; if she would be like all the wicked stepmothers in the stories," he writes.
Harry acknowledges using cocaine on several occasions when he was a teenager, saying he was "willing to try almost anything that would alter the pre-established order".
But he adds: "It wasn't very fun, and it didn't make me feel especially happy as seems to happen to others, but it did make me feel different, and that was my main objective. To feel. To be different."
Harry recounts losing his virginity in a field behind a busy pub, to an "older lady" who "loved horses very much".
But he says it was a "humiliating episode" and rued it happening in the open, where they might have been seen.
'Your mother is with you'
Harry recounts seeking help since moving to California from a woman who "claimed to have 'powers'" and an ability to relay messages from the dead.
The Duke of Sussex does not describe her as a psychic or medium but writes that he felt "an energy around her" as soon as they met.
"Your mother is with you... right now," he says the woman told him. Harry says his neck grew warm and his eyes watered.
"You're living the life she couldn't. You're living the life she wanted for you," she said.