Royal author Robert Jobson insisted the once-close brothers “ought to” make it up. William and Harry, who has quit royal duties for a new life in America with wife Meghan Markle, have faced ongoing claims of a rift.
Mr Jobson said Diana, who died in a Paris car crash in 1997, would want her sons to do a television interview to put an end to the speculation over their relationship.
The royal expert told Us Weekly: “And I think that, you know, their mother, if [she] was alive, I trust [she would] bang their heads together and say, ‘Do a television makeup. Do a television interview and just stop the stories.’
“Because we’re never going to stop talking about them unless they do something about it.”
Mr Jobson added that he is “sure the Prince of Wales is upset”.
He said: “We all fall out with our siblings [now and] again, but to continue into a feud is a little, unlike Harry, actually. The character that I knew, he was quite relaxed, actually. And I think he wouldn’t have wanted to continue this. So, I don’t understand what the problem is.”
Mr Jobson said there “must be something really deep down that’s offended [Harry]”.
He added: “I personally think that [Harry] should do it whether or not they are able to [fully forgive].
“I mean, it seems that they probably allowed things to go on too far.”
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The statue is set to be unveiled in the Sunken Garden of Kensington Palace on what would have been Diana’s 60th birthday on July 1.
Royal author Omid Scobie claimed any tensions have not stopped William and Harry planning the memorial.
The royal biographer told OK! magazine: “Despite their differences, Harry and William have been equally involved in planning the memorial and sometimes have been in on the same calls or emails.
“No matter what has happened between them, they would never let anything stand in the way of this.
“It would be wrong to say their relationship is healed, but 2020 forced them to have many conversations.
“For instance when Charles was sick with Covid and then William. And when Meghan suffered a miscarriage.
“The focus on family pushes them forward and while progress with their healing is still minimal, it’s moving in the right direction.”
But last month, Harry’s biographer Angela Levin warned the event is likely to be “a bit awkward” for the brothers.