William and Harry urged to reconcile at unveiling of Diana's statue - 'If this doesn't do it, nothing will'

Watch: Princess Diana's Incredible Legacy Remembered

Prince William and Prince Harry will "know the answer"about what their mother Diana would have wanted as all eyes are on them to heal their rift after unveiling a statue of her at Kensington Palace.

The brothers, who have had a strained relationship over the last 18 months, are uniting at the Sunken Garden to reveal the statue they commissioned of their late mother to the world on what would have been her 60th birthday.

Ahead of the event, Paul Burrell, her former butler, said he hoped it would bring the brothers back together.

Speaking of the rift healing he told ITV's Good Morning Britain: "If this poignant day doesn’t do it, then nothing will because if they stand there in front of their mother’s image and just think and consider what she would have wanted and what she would have said to her boys, then they will know the answer to that question.”

He also suggested the Duchess of Cambridge, who won't be at the ceremony, will hold a key role. Kate, 39, has previously been dubbed a peacemaker, and it was her manoeuvring at the funeral of Prince Philip that ensured William and Harry spoke as they walked back to Windsor Castle.

Read more: Princes William and Harry to unite to unveil statue of Diana

Burrell continued: "I think personally they will go back into Kensington Palace after the ceremony which will be very small, very quick - they will go back to Kensington Palace and they will meet up with Kate indoors and I think Kate holds the key to this, I think she is the peacekeeper, she is the person who is going to pull these boys back together. There’s a lot of work today, because there’s a lot of things that have been said.

"[Kate’s] the answer because she is standing between both boys seeing this unfold and she knows that they’re both hurting, she knows they both want this to work… She knows that William wants this to work out. Those boys should be stood together, their mother would want them to stand together."

Harry has been outspoken about the monarchy in recent months, even suggesting that his brother and father, Prince Charles, were "trapped" in the institution.

But speaking of Diana, Burrell added: “Her idea would be that Harry would support William when he came to the throne, it was his job as his brother and they would be there together helping each other.

"Don’t forget Diana was a great monarchist, she supported the Queen and Prince Philip and she loved the institution into which the boys were born. She was very proud of her boys being members of the Royal Family and she would want them to continue that way, and I believe there will be a role for Harry in the future when William becomes King."

Long term royal correspondent Robert Jobson told Good Morning America it was "baby steps" for the brothers, but hoped they would reconcile.

He said: "I remember when they were little boys, they were always hugging, great mates. Wouldn't it be lovely if they would jut embrace and let bygones be bygones?"

He said being with Diana's sisters might be helpful because they have been arbiters in the past.

He added: "There family occasions are moments when they can get a moment to chat.

"Just standing there and looking at that statue... in the garden where they used to play as kids, maybe that will be the catalyst to heal this rift."

Read more: Prince Harry says two children 'is a juggle' but Lilibet is 'chilled'

The rift between Harry and William has been reported as starting when Harry was dating Meghan, with William encouraging him not to rush the relationship.

It was reported to have worsened when William saw allegations of bullying by Meghan of palace staff, which she has since denied.

The situation has not been helped by the Duke and Duchess of Sussex moving to California, her home state, and stepping back from royal duties, or by a series of interviews they have done since leaving the fold.

William was forced to deny being part of a "racist family" after Harry and Meghan said there were questions raised about the colour of their future children's skin.

Robert Lacey, author of Battle of Brothers which detailed their rift, said in The Times of the ceremony: "All those present will be thinking about nothing but her — what she meant as a figure of modern British history and what she meant to them personally. And surely, when it comes to personal feelings, Diana cannot possibly have wished or imagined that her two beloved sons should end up in such discord."

He added: "William and Harry have worked harder than people realise to raise this statue in their mother’s honour. Now they have the chance to add to that tribute with the greater initiative of a genuinely loving and forgiving brotherly relationship."

The unveiling takes place on Thursday afternoon in Kensington Palace.

Watch: How Princess Diana's death changed the British monarchy