Prince William and his brother Prince Harry will for the first time discuss the upset they still feel after their mother’s death 20 years ago in a documentary to air tonight, Monday, July 24.

In the documentary about Diana, Princess of Wales, William and Harry speak of their regret over their last phone call to their mother and how brief it was; wishing they had spoken to her for longer. Harry described how it was his turn to chat to his mum after his brother. He said: “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.”

A documentary exploring Prince William and Prince Harry's lives with their mother is being aired tonight

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

On the day Diana was killed William and Harry were at the Queen’s private Scottish home of Balmoral playing with their cousins.

Reminiscing over their time with their mother, Prince 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 documentary, Prince Harry recalls how he still remembers his mother’s laugh. They talk openly about their mother’s 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….”

Stills from Princess Diana: Her Legacy

In the documentary The Duke of Cambridge confesses he tells his two children, Diana’s grandchildren Prince George and Princess Charlotte that they have two grandmothers. William says “his mother would have loved Charlotte and George to bits”.

On what would have been Diana’s birthday – July 1 – William, Kate and Harry, joined by Prince George and 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”.

It will also feature the moment when, on December 9, 1992, the then Prime Minister John Major announced in the House of Commons the separation of Prince Charles and Diana.

The documentary, Diana, Our Mother: Her Life and Legacy will be screen at 9pm tonight, Monday, July 24, on ITV.