Princess Diana once made Prince William and Prince Harry "promise to be best friends", a former friend has claimed

In an updated edition of royal historian Robert Lacey's book 'Battle of Brothers', Simmons says that Diana – who tragically passed away in a Paris car crash in 1997 – told William and Harry, who she shared with former husband Prince Charles: "You must promise me that you will always be each other's best friends.

"And never let anyone come between you."

Lacey adds: "Both boys promised they would keep to that, Simmons remembers.

"They high-fived each other and gave their mother a big cuddle. Anybody would have melted at the sight. Then they went out to play soccer."

The Duke of Cambridge and the Duke of Sussex will reunite at the unveiling of Diana's statue at Kensington Palace on July 1, to mark what would have been her 60th birthday.

Harry – who relocated to the US with his wife Meghan, Duchess of Sussex, after stepping down as a senior royal last year – will stay with Princess Eugenie at Frogmore Cottage in Windsor, where he is able to quarantine for the required five days before being tested for coronavirus.

A source told The Sun newspaper of the family's plans: "The building was originally five different cottages before it was given to Harry and Meghan. They spent a lot of money getting it into shape for their family - before quitting for California. Eugenie and Jack were handed the keys last year and told they could use it while Harry and Meghan were in America.

"The cottage is now literally split into two, meaning Harry can isolate in one half of the house without ever coming into contact with his cousin and her family. It has been set up for his return after last-minute plans seemed to work when he was back for the Duke of Edinburgh’s funeral in April."