The heartbreaking promise Prince William and Prince Harry made to Princess Diana before her death

Prince William and Prince Harry both made a promise to their mother to always remain "best friends" and "never let anyone come between them" a new book reveals.

• Prince Harry and Prince William reportedly promised Princess Diana they'd always remain "best friends" according to her psychic friend, Simone Simmons.
• The relationship between the brothers is said to be tense since Prince Harry and Meghan Markle's Oprah interview.
• This follows royal news that Prince Charles chose to speak out on this emotive topic with "unusually strong words"

Princess Diana once made Prince William and Prince Harry "promise to be best friends" forever, according to a psychic friend of hers. 

Simone Simmons used to meet with Princess Diana before her tragic death in 1997, and has since revealed the one wish she made her sons promise to keep.

Simone's quotes feature in Robert Lacey's updated version of his book, Battle of Brothers, in which she claims Diana said, "You must promise me that you will always be each other’s best friends."

"And never let anyone come between you."

Of course that promise is even more devastating now that the brothers are arguably worlds apart following Prince Harry's decision to 'step back' as senior members of the royal family. 

After admitting they were on "different paths," Prince William and Prince Harry are now embroiled in a full-scale feud which royal experts say has a "long way to go" before being healed.

In his book, Robert Lacey goes on to recall another of Simone's memories.

"Both boys promised they would keep to that. They high-fived each other and gave their mother a big cuddle."

The close bond she talks of is a far cry from the Duke of Cambridge and the Duke of Sussex's current relationship, which is said to be tense after the Duke launched a scathing attack on the institution in a bombshell interview with Oprah Winfrey.

Despite their differences, the boys are expected to put on a "good show" in a few weeks as they finally unveil a memorial statue for Princess Diana in the Sunken Garden at Buckingham Palace. 

It will be the first time the brothers have seen each other since Prince Philip's funeral.