LONDON (AFP) - Britain's Prince William and Prince Harry have revealed they talked to their mother Princess Diana on the day she died and that the "short" conversation now weighs "heavily" on their mind.

"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," Prince Harry told ITV for a documentary to commemorate their mother.

Prince William was just 15 and his brother Harry 12 when their mother and her boyfriend Dodi Al-Fayed were killed in August 1997 after the car they were in crashed in a tunnel in central Paris as it was being pursued at high speed by press photographers.

Their French chauffeur Henri Paul, who was later found to be over the legal blood alcohol limit, also died.

The two princes were in Balmoral, the queen's residence in the Scottish Highlands, and William had earlier told Diana of the "very good time" they were having.

"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," William told ITV.

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

To mark the 20th anniversary of her death, the princes announced earlier this year they were setting up a committee to raise funds to pay for a statue of Diana, who was known as the Princess of Wales.

The statue is to be erected in the public gardens of Kensington Palace in London, where she lived.

The documentary, entitled Diana, Our Mother: Her Life And Legacy, is to be aired in the UK on Monday (July 24).

"I think you get the private Diana," Nick Kent, the film's executive producer, told Reuters. "Nobody has ever told this story from the point of view of the two people who knew her better than anyone else, and loved her the most: her sons."

The princes recall their mother's sense of humour, with Prince Harry describing her as "one of the naughtiest parents". They also recall the pain of their parents' divorce, and how they dealt with the news of her death and its aftermath.

While the film addresses such aspect of Diana's life as her charity work involving HIV and landmines, it shies away from more controversial issues, such as extra-marital affairs.

According to the makers, however, the British royals were very open, and did not put any subject off limits. Rather, they wanted to cover new ground and make a different type of film.

"What we had in mind is that in years to come, Princes William and Prince Harry would be happy to show this film to their own children and say this is who your grandmother was", Kent said.