Prince William's 'promise' to his mother Diana

Prince William's close relationship with his mother, Diana, is well-known and now it's been claimed he made a very special promise to her, one year before she died.

Diana's former butler, Paul Burrell, says the Princess was unhappy about losing one of her titles following her separation from Prince Charles, according to the Mirror.

After the couple divorced, Diana would no longer be known as Her Royal Highness.

Her Majesty the Queen was reportedly fine with Diana keeping the HRH status, instead Prince Charles insisted it be stripped from her name.

It meant she now had to curtsy to her sons, William and Harry, and was further distanced from the royal family.

William was just 14 at the time of his parents' divorce and promised his mother to restore her title.

"Don't worry Mummy, I will give it back to you one day when I am king," Prince William told his mother.

Burrell makes the claim in his book, A Royal Duty.

It's believed William made the promise to Diana when he was a student at Eton College.

Tragically, the young royal was never able to follow through with his vow.

Diana was killed in a car accident in Paris in 1997.

William was just 15-years-old.

Burrell's relationship with the Royal Family has been rocky since Diana's death, with Burrell frequently releasing intimate details about what he claims went on behind palace walls.

Last year, Burrell claimed he'd not spoken to William or Harry since their mother died.