Prince Harry has said his father the King carried a "pitiful" teddy bear from his troubled school days as an adult.
The Duke of Sussex's highly-anticipated memoir, Spare, has gone on sale today, with a number of bookshops opening at midnight to meet demand.
Many of the book's explosive claims about the Royal Family have already been reported on after a Spanish edition of the book was accidentally released early.
The revelations have sparked criticism aimed at Harry from those who believe he is airing unnecessary private details about his family in public.
Harry believes that he is setting the record straight after what he says his years and years of family members and royal aides leaking details about his own personal life to the press.
In the book, Harry describes what he says were the troubled schooldays of his father at Gordonstoun, a boarding school where Charles was "horrendously bullied".
Harry says his father told him he was sent away for his education to "toughen him up", adding: "I remember him murmuring ominously: I nearly didn’t survive. How had he?"
Harry goes on to describe Charles' teddy bear.
"Head down, clutching his teddy bear, which he still owned years later. Teddy went everywhere with Pa. It was a pitiful object, with broken arms and dangly threads, holes patched up here and there.
"It looked, I imagined, like Pa might have after the bullies had finished with him. Teddy expressed eloquently, better than Pa ever could, the essential loneliness of his childhood."
Watch: Prince Harry's memoir divides public opinion
Harry adds that Charles "deserved a proper companion", not a mere teddy bear, which is why he and his brother William promised that they would welcome Camilla - now the Queen consort - into the family.
Later, the duke claims he and William "begged" their father not to marry Camilla, who Harry describes as playing a "pivotal role" in unravelling Charles' previous marriage to the late Princess Diana.
Harry's tell-all book, ghost-written by novelist J. R. Moehringer, includes other details about Charles' life that the King might want to keep private, including those of an unusual morning ritual.
"Open the wrong door and you might burst in on Pa while his valet was helping him dress," the duke recalls.
"Worse, you might blunder in as he was doing his headstands. Prescribed by his physio, these exercises were the only effective remedy for the constant pain in Pa's neck and back. Old polo injuries, mostly.
"He performed them daily, in just a pair of boxers, propped against a door or hanging from a bar like a skilled acrobat.
"If you set one little finger on the knob you'd hear him begging from the other side: No! No! Don't open! Please God don't open."
Harry has defended his decision to publish a memoir, claiming that after 38 years of "spin and distortion" in the media, he finally had an opportunity to "own my story".
However, he has faced criticism for sharing details of his family that have nothing to do with setting the record straight - all while complaining about Royal Family members leaking details about him to the press.
In particular, he was accused of "jaw-dropping hypocrisy" following an interview on Sunday with ITV News' Tom Bradby, in which he said he hoped any details of conversations he had with his father and brother about the book would remain private.
This is despite the duke using his memoir, a Netflix documentary series, and a series of interviews, including a sit-down with Oprah Winfrey in 2001, to air grievances about the family.
Harry accused members of his family of "getting into the bed with the devil" and planting stories about him and Meghan in the tabloid press in order to bolster their own public image.
He told Bradby he had fled Britain with his family for California in 2020 "fearing for our lives" and said he no longer recognised his father or his elder brother.
"After many, many years of lies being told about me and my family, there comes a point where, going back to the relationship between certain members of the family and the tabloid press, those certain members have decided to get in the bed with the devil ... to rehabilitate their image," he said.
"The moment that that rehabilitation comes at the detriment of others, me, other members of my family, then that's where I draw the line."
Sharing her take on the ITV interview, author Caitlin Moran wrote: "The royal family are so scared of the media they regularly chucked Harry & Meghan under the bus, so they would look good by comparison.
"And now - given that quitting & moving to the US didn't stop it - he feels he has nothing to lose by revealing it."