The Duke of Cambridge and Prince Harry have revealed their last conversation with their mother was a brief phone call on the day she died that now weighs “heavily” on William’s mind.

In a documentary about Diana, Princess of Wales, William and Harry speak of the remorse they feel at how short their final chat with their mum was, with Harry confessing it is something he will regret “for the rest of my life”.

William and Harry at their mother's funeral. (Adam Butler/PA)William and Harry at their mother’s funeral. (Adam Butler/PA)

But looking back at the life of the princess, whose death 20 years ago shocked the world, Harry said “to myself and William she was just the best mother ever” who “brought a breath of fresh air to everything she did”.

In the programme that chronicles Diana’s personal journey, her campaigns supporting the homeless, Aids victims, and banning landmines, and her death, Harry says he has only cried twice for his mother – once at her funeral and during an undisclosed occasion.

William was 15-years-old and Harry just 12 when their mother was killed in a car crash in Paris on August 31 1997.

Diana with the young princes. (Martin Keene/PA)Diana with the young princes. (Martin Keene/PA)

On the day she was killed William described the “very good time” they were having at Balmoral, the Queen’s private Scottish home, where the royal brothers were playing with their cousins.

William, interviewed with his brother at Kensington Place for the ITV documentary, said: “Harry and I were in a desperate rush to say goodbye, you know ‘see you later’…if I’d known now obviously what was going to happen I wouldn’t have been so blase about it and everything else.

“But that phone call sticks in my mind, quite heavily.”

A young William and Harry. (The Duke of Cambridge and Prince/PA)A young William and Harry. (The Duke of Cambridge and Prince/PA)

Asked if he remembers what his mother said he replies “I do” but does not divulge the conversation.

Harry described how it was his turn to chat to mum after his brother: “It was her speaking from Paris, I can’t really necessarily remember what I said but all I do remember is probably regretting for the rest of my life how short the phone call was.

“Looking back on it now it’s incredibly hard, I’ll have to sort of deal with that for the rest of my life. Not knowing that was the last time I was going to speak to my mum, how differently that conversation would have panned out if I’d had even the slightest inkling her life was going to be taken that night.”

On Diana’s birthday – July 1 – William, Kate and Harry, joined by Prince George and Princess Charlotte, attended a service to re-dedicate her grave at Althorp house, where she was buried on an island.

Harry said: “The first time I cried was at the funeral on the island and probably only since then maybe once. So there’s a lot of grief that still needs to be let out.”