The Duke of Cambridge and Prince Harry have revealed details of the final conversation they had with their mother almost 20 years after her death.

Their final words with her were exchanged in a brief phone call on the day she died that now weighs heavily on William’s mind, he said in a new interview.

In a documentary about Diana, Princess of Wales, the brothers speak of their lasting regret at how short their final chat with their mother was, with Harry confessing it is something he will regret “for the rest of my life”.

William was 15 and Harry 12 when their 36-year-old mother was killed in a car crash in Paris on 31 August 1997, with the image of the two young princes walking behind their mother’s coffin at her funeral one of the most enduring of the 20th century.

But looking back at the life of the princess, whose death 20 years ago shocked the world, Harry, now aged 32, 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 documenting Diana’s personal journey, her campaigns supporting the homeless, Aids victims, and banning landmines, and her death, Harry reveals he has cried just twice for his mother – once at her funeral and on another occasion that he would not disclose.

Diana with her sons in April 1992 (PA)

William – who at 35 is now just a year younger than Diana was when she died – 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 when their mother called.

William, interviewed with his brother at Kensington Place for the ITV documentary, said he spoke to his mother first.

“Harry and I were in a desperate rush to say goodbye, you know, ‘see you later’ ... f 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, William replied “I do”, but did not disclose details of the conversation.

Harry then took his turn to talk to their mother.

“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,” he said.

A picture from Diana’s personal photo album of her William. She was pregnant with Harry (Kensington Palace)

“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 what would have been Diana’s 56th birthday, 1 July, William, Kate and Harry, joined by Prince George and Princess Charlotte, attended a service to rededicate her grave at her childhood home, Althorp House, where she was buried on an island.

Harry, who has spoken publicly about his own mental health battles, 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 vey public marriage break-up with Prince Charles was a defining moment in her life, and the documentary features the moment when, in December 1992, the then-Prime Minister John Major announced to the Commons the couple had agreed to separate.

Previously unseen family photos of the royal brothers taken by their mother are shown in the programme with William and Harry filmed trawling through the albums compiled by their mother.

The Diana theories in full Show all 10 1 /10 The Diana theories in full The Diana theories in full MI6 Assassination Getty The Diana theories in full The Crash as Suicide This theory states that Diana, sick of a life in the public eye, decided to fake her own death – a theory fuelled by Di's mysterious comment to Daily Mail reporter Richard Kray earlier that day that she was about to 'completely withdraw' from public life. Though Mercedes auto experts claim that surviving the 121mph crash would have been near impossible, conspiracy theorists argue that either the car wasn't going 121mph after all, or that Diana and Dodi weren't in the car at all, and that the crash was faked by the army-trained bodyguard Trevor Rees-Jones. Getty The Diana theories in full Her colourful love life Scandal over Diana's love-life continue even now. Her list of conquests, some more believable than others, include: Prince Charles, Dodi Al Fayed, the doctor Hasnat Khan, infamous love-rat James Hewitt, England rugby star Will Carling, bodyguard Barry Mannakee, PR executive James Gilbey, art dealer Oliver Hoare… and now Valéry Giscard d'Estaing. There are also rumours of her flirtations with Tony Blair during a dinner party at No.10. Getty The Diana theories in full The mysterious Fiat Uno… Traces of paint left on the bodywork of Diana and Dodi's Mercedes fuelled claims that a white Fiat Uno was used by the security services to cause the crash. The lack of CCTV of the crash, in spite of the 10 cameras along the route the car took, has fuelled this theory. Fayed claimed the car belonged to French photojournalist James Andanson, who later committed suicide – either due to his guilt or in a security service assassination plot. Andanson had a solid alibi proving he was at home 175 miles from Paris on the night of the crash, and both the French police and Operation Paget concluded that it was extremely unlikely the car was Andanson's. The vehicle has never been found - Andanson's body was though, in a burnt out BMW in a forest in the south of France. Wikipedia The Diana theories in full Burrell’s Ring Stealing Antics An old bodyguard of Paul Burrell claims that the former butler stole a ring from Diana's body shortly after her death. Apparently this ring was a ‘friendship’ ring from Dodi Al Fayed. The suitably named Mark Faux claims that this was one of the many items Burrell stole during his time working for the royals, including some Buckingham Palace-headed writing paper. Getty The Diana theories in full Diana's penchant for hairy backs Adding a little more spice to the inquiry, Lord Stevens, the ex-Metropolitan Police Commissioner, allegedly told the court of Diana's fondness for men with hairy backs. Harder to ascertain than the truth of this allegation is the precise hairiness of each of Diana's lovers' backs. Getty The Diana theories in full Flashlight theory Following on from the MI6 conspiracy theory, Tomlinson also suggested that the alleged bright white flash of light just before the car entered the tunnel was a cunning tactic by MI6 to blind the driver of the vehicle. The police found no evidence that this tactic had ever been used by MI6, yet Tomlinson claimed it formed part of MI6 agents' training in the early 1990's and was used to blind helicopter pilots. This conspiracy theory was backed up by the testimony of nearby driver François Levistre, though his wife, who had been seated next to him as he drove the car, refuted the claims, and Levistre himself had a history of criminal offences involving dishonesty. Getty The Diana theories in full A little Dodi-Diana on the way Some claim that a motive for the alleged murder of Diana and Dodi was their upcoming engagement and the possibility that Diana was pregnant with Dodi's child. These claims persist in spite of the post-mortem examination having found no evidence of Diana's pregnancy, with Mohammed Al Fayed claiming that her body was intentionally embalmed soon after her death in order to thwart any post mortem results. Dodi did buy a ring on the day of their deaths, though it is unknown whether this was intended as an engagement ring. Mohammed Al Fayed claimed after the crash that the couple had been planning to announce their engagement on the 1st September 1997, just days afterwards. Theorists claim that the royal family’s apparent racism would have rendered this Diana-Dodi union intolerable, and thus Diana had to go. Reuters The Diana theories in full Dodi Assassination Attempt Another theory claims that Diana's death was merely a convenient cover for an assassination plot against Dodi, orchestrated by business enemies of Dodi and his father Mohammed. Theorists point to Mohammed Al Fayed's colourful career history, and the idea that killing Dodi would have been the ultimate revenge on Mohammed by one of the many enemies he had picked up on his path to wealth and power. Getty The Diana theories in full The biggest scandal of them all Arguably the biggest conspiracy surrounding Diana, as argued by Joan Smith, is the huge cost to the taxpayer of the Diana inquest, as a result of the proliferation of wild allegations about her life and death. Getty

In lighter moments, they discuss 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 on Monday at 9pm.