Royal Biographer Angela Levin has warned the King and Queen following their meeting with Prince Harry at Clarence House.
The Duke of Sussex flew overnight on Monday to his homeland of the UK after the King announced to the public that he has been diagnosed with cancer.
A statement from Buckingham Palace read that the King is to receive "scheduled, regular treatment" following his diagnosis, and the monarch remains "wholly positive" about his health.
Prince Harry made the swift trip to see his family, landing at Heathrow Airport on Tuesday.
Angela Levin says the King and Queen 'can't trust' Prince Harry
PA / GB News
The Duke was then granted a police escort into central London, where the estranged Prince met with the King at Clarence House.
It is claimed that the meeting lasted a "maximum of 45 minutes". A helicopter was then seen leaving Buckingham Palace as the King and Queen headed to their Sandringham estate.
Prince William, who's rocky relationship with Prince Harry has not yet been reconciled, has reportedly "not made any plans" to reunite with his brother whilst he is in the UK.
Prince Harry travelled to the UK alone, leaving wife Meghan Markle and children Archie and Lilibet in California.
Prince Harry visited the King for '45 minutes' at Clarence House following the King's cancer diagnosis
PA
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Speaking to GB News, biographer Angela Levin said Prince Harry has "lost" the trust of the Royal Family since stepping down from duties in 2020.
Levin explained: "Once you can't trust somebody, it's very, very difficult to trust them again because you feel he's either going to try and make money out of it or he's going to find a small error that he then can make a huge complaint about.
"He will not compromise. He used to be somebody who would negotiate with other people in the Royal family, he was known for putting things right."
She added: "But now he's known for putting things wrong, and he just will do as much as he can to make sure he's right and his father is not."
King Charles and Queen Camilla were driven to Sandringham following his reunion with Prince Harry
PA
Levin criticised Prince Harry for the nature of his trip and said the speed of which he flew to the UK following the announcement was "very him".
She stated: "It shows a lot about him is the way he's arrived here. If you know you have a father who you haven't seen for a long time, who you know has just heard he's got cancer, I think you should ring up.
"You should say 'I would love to come, but I don't want to come at a bad time, I don't want to upset him or in any way, can you tell me what would suit him?'
"This trip seems to be one hundred per cent Harry. He flies off in a rush, he gets here when the helicopter's all ready to take the King and Queen away, and then he wants to talk it. It's all about him."