The Royal brothers are expected to set aside their disagreements for the unveiling of Princess Diana’s monument on July 1 at Kensington Palace to commemorate their mother’s 60th birthday.

According to a royal expert, Prince Harry will never be able to mend his relationship with his elder brother as long as he is “under the thumb”

Hugo Vickers, author of many books on the royals, said that Harry had “thrown in his lot with his wife” and that slagging off his family” was counterproductive.

He further stated that any effort by the brothers to reconcile in London would result in Harry “having to answer for it when he returns to his wife in Los Angeles”.

“I don’t know what they can do under the present circumstances,” he told The Express.

“Harry has got to wake up to what’s going on. It’s a very unpleasant situation and I don’t think it’s going to be the right moment this week.

“Harry is so under the thumb of Meghan it is not really possible until he emerges from that.”

He went on to say that the brothers could connect by videolink or phone, but that neither Harry nor Meghan wanted to.

He said: “He has thrown in his lot with his wife.

“That’s it, isn’t it? Slagging off his family didn’t help anybody. It’s a dreadful situation.”

Mr Vickers also believes Prince Charles will not intervene, adding:

“You’ve got to remember that Harry has to return to his wife in Los Angeles. If he starts reconciling, he will get his head bitten off, won’t he?”

There are concerns that the tension may overshadow the unveiling of the new monument, but other sources believe the two will concentrate on their mother’s legacy.

“The story will not be the actual unveiling of the statue but of the two boys together, that’s what every­­one will be focusing on,” Mr Vickers added.

Last month, the Duke of Sussex admitted that coming to London following Diana’s death was a “trigger” for him, while he addressed his own traumas in an on-screen therapy session.

Another Royal author, Richard Fitzwilliams, believes the brothers will put on a polished public show.

He said: “I think they will appear publicly mutually supportive.

“But with Harry’s recent interviews and with so many rumours circulating, things need to become a lot calmer.”

Prince Charles is in Scotland ahead of the inauguration of Diana’s monument due to a scheduling conflict.

A source said: “Charles will leave the boys to it. Harry will need to quarantine for at least five days when he lands in England, most likely at Frogmore Cottage in Windsor.

“Charles has made it quite clear he will not be around beyond that because he is going to Scotland. There is no planned meeting between the three of them.”

The allegations come as Robert Lacey, author of the new book Battle of Brothers, which chronicles the princes’ feud, thinks tensions are “not likely to end any time soon” despite the fact that they were spotted talking following their grandpa Prince Philip’s funeral in April.