A worried Prince William told Prince Harry that he believed he had been “brainwashed” by the therapist he was seeing, the younger brother claims.
In Harry’s new memoir, Spare, the Duke of Sussex outlines the moment he claims the Duke of Cambridge said he wanted to come along to one of his confidential therapy sessions.
William wanted to do this as he was worried about what his younger brother was being exposed to, Harry claims.
Harry also said that William thought he was unwell and "unwise", as he planned to move abroad with his family and leave behind his life as a working Royal.
William also turned down Harry's offer of a joint therapy session with him, as he suggested the idea in order for the brothers to mend their relationship, telling William it would "be good for you. Good for us," he claims.
In the excerpt from Harry’s new book, reported by the Daily Mail, Harry accuses William of believing "I was unwell, which meant I was unwise", as he made preparations to step away from the Royal family for a new life in the US.
The Prince of Wales has been vocal in his public support of mental health causes, which he has made one of his key missions.
The latest claims to be revealed from Harry's explosive memoir, Spare, come ahead of the younger Prince's interview with ITV tonight.
In a clip from Harry: The Interview, which will be broadcast at 9pm on ITV1 and ITVX on Sunday, the Duke of Sussex tells presenter Tom Bradby about his memories of meeting people following the death of Princess Diana in 1997 when he was 12 years old.
"Everyone knows where they were and what they were doing the night my mother died," he says.
"I cried once, at the burial, and you know I go into detail about how strange it was and how actually there was some guilt that I felt, and I think William felt as well, by walking around the outside of Kensington Palace."
Harry described feeling the mourners' tears on their hands when he shook them.
"There were 50,000 bouquets of flowers to our mother and there we were shaking people's hands, smiling," he says.
"I've seen the videos, right, I looked back over it all. And the wet hands that we were shaking, we couldn't understand why their hands were wet, but it was all the tears that they were wiping away."
It was previously reported Harry also opens up about the rift between himself and his brother in his book, claiming that Prince William “grabbed him by the collar and knocked him to the floor”, as the pair brawled over Meghan Markle.
Harry alleges the confrontation took place in London in 2019, which left him with cuts and scratches.
William reportedly arrived to Nottingham Cottage to speak too Harry about their relationship, which quickly turned into an argument.
William allegedly claimed he was trying to help, before the pair began to brawl.
Harry wrote: "It all happened so fast. So very fast. He grabbed me by the collar, ripping my necklace, and he knocked me to the floor. 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.”
A source has reported that Prince William is "burning with anger” at the revelations in Spare, but would "never retaliate".
The source, who is said to be a close friend of William, described the future king as a “sitting duck”, saying: “He won’t retaliate, he never would, because he’s dignified and unbelievably loyal.
"William is a sitting duck because Harry knows he isn’t going to retaliate. How many shots can you take at a sitting duck?
“It’s cruel, cowardly and so sad for William to keep taking the punches. He’s keeping quiet for the good of his family and the country.
"He’s handling it so well on the outside — inside he’s burning.”
