WATCH: William and Harry's remorse over last brief chat with Diana
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 TV documentary tonight 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".
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.
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."
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."
Diana's marriage break-up was a major turning point in her life and the princess's close friend Harry Herbert talked to the Queen about her emotional state following a Balmoral lunch, because the monarch "was so worried about Diana".
He said: "The Queen wanted to know how was Diana feeling and was it as bad as it was, and it was a sad discussion, a sad moment really - that was everything at its worst."
The documentary, which is screened on Monday, went on to feature the moment when, on December 9, 1992, the then Prime Minister John Major announced to the Commons the couple had agreed to separate.
Unseen family photos of the royal brothers taken by their mother are shown in the programme with William and Harry seen trawling through the albums compiled byDiana.
In lighter moments the princess's sons talk about her sense of humour with Harry saying: "Our mother was a total kid through and through, when everybody says to me 'so she was fun, give us an example' all I can hear is her laugh in my head..."
Reflecting on the 20 years since his mother's death, Harry said: "It has been hard and it will continue to be hard, there's not a day William and I don't wish that she was still around and we wonder what kind of mother she would be now, and what kind of a public role she would have and what a difference she would be making."
Diana never saw her work to help outlaw landmines come to fruition as she died before the international treaty to ban the military weapons was signed.
But Harry described how he found letters on the subject dated the day of her death, but never sent: "About a month ago I found a whole series of letters. Letters that she was supposed to top and tail that were dated 31st of August that were sitting on her desk here.
"She knew exactly what needed to be done, she was writing letters to certain people to say 'right, this is what needs to happen in order for this whole tidal wave to change".
The documentary, Diana, Our Mother: Her Life and Legacy will be screened on ITV tonight at 9pm.