The Sussexes are preparing to return to the United Kingdom for an Invictus Games event in July. While King Charles has reportedly mellowed over recent months, the rift between Harry, 41, and Prince William, 43, persists.

According to reports by Heat, as William readies himself to assume the throne, Harry has allegedly made some caustic remarks about his brother in private. An insider told the publication, "Harry is very snarky about William, he thinks he's drooling over the power and going to be a total nightmare once he gets it."

The source added, "He says people don't know the real William the way he does, and once they see the truth they'll be shocked."

This follows revelations that Prince William is irritated that King Charles extended an invitation to Prince Harry and Meghan to stay at the royal estate this summer.

"He is not shy about saying that William doesn't have the temperament to be King. He says he's too vindictive and cruel to be a good ruler, not to mention incredibly stubborn and hot-headed," the insider said.

They continued, "Everyone knows William likes being in charge but struggles when things don't go his way, he's been that way since they were kids, very touchy and quick to explode, according to Harry."

Another source told Heat that "Harry is predicting that once William is handed the keys to the Kingdom it will be an epic disaster, he doesn't come out and say he's rooting for him to fail, but it does seem that way."

Princess Diana's former butler and close friend Paul Burrell has previously stated that Diana would have been "devastated" by their rift.

"If their mother was here today, she would bring them back together...It was very important to her that they stayed close. [Princess Diana's] vision was that the two of them would be inseparable, they would grow together and be two princes, one would be king one day but the prince supporting the king," he said, per Heat.

Paul further noted that Harry was conscious of all the exchanges between William and palace staff during their upbringing, stating, "Harry understood his brother would one day be King because Harry listened to people whispering in William's ear all his childhood years, 'One day, you're going to be king. You have to sit up straight because you're going to be king. You have to know how to eat properly with a knife and fork because you're going to be king.'"

Biographer Phil Dampier suggested that King Charles "wants a reconciliation" with Harry, though this would probably take place privately to prevent the encounter from descending "into a circus."

While Mr Dampier speculated Charles and Harry might meet, he acknowledged uncertainty over whether Queen Camilla would wish to reconnect with her stepson.

He told the Daily Mail, "It wouldn't surprise me if she has some family commitment to go to and avoids them. She will feel she was badly treated in Harry's book Spare."