Prince Harry claims he and his brother Prince William "begged" King Charles not to marry Queen Consort Camilla.
The 38-year-old royal admitted he and his older sibling were aware the then-Camilla Parker Bowles was the "other woman" and both had meetings with her before she joined the family, but were concerned she would become their "wicked stepmother".
Admitting he was unsure if Camilla would one day be his "wicked stepmother", Harry claimed he and his brother were willing to forgive her in "their hearts" if she could make their father happy.
However, he added: "We saw that like us, he wasn't.
"We could recognise the absent glances, the empty sighs, the frustration always visible on his face."
Harry claimed he and William promised they would welcome Camilla into the family but "begged" Charles not to marry her after the death of their mother Princess Diana.
He wrote: "William and I promised our dad that we would welcome Camilla into the family.
"The only thing we asked in exchange is that he didn’t marry her.
"We begged him - you do not need to get married for a second time."
He alleged the king didn't respond, and a campaign began for the couple to marry and for Camilla to one day be queen.
In an extract from his memoir 'Spare' obtained and translated by The Sun newspaper, who had obtained a copy of the book from Spain, where it has mistakenly gone on sale early, Harry compared meeting his future stepmother for the first time to avoiding pain from getting an injection.
He wrote: "This is nothing. Close your eyes and you won't even feel it..
But the Duke of Sussex - who stepped down from royal duties to start a new life in California with wife Meghan, Duchess of Sussex, in 2020 - claimed Camilla seemed "bored" by their interaction because it was "pure formality" as he wasn't the heir, though they had a small chat about horses.
He wrote: "None of us were particularly worried about each others opinion on the other.
"She was not my mother and I was not her major obstacle, in other words I was not the HEIR.
"That scene with me was pure formality."
In another revelation, Harry accused Camilla of leaking details of a conversation with William to the media.
He also told how Charles "nearly never made it" after being bullied at school and claimed his dad always carried a teddy bear with him as a coping mechanism.