LONDON -- It was a typical phone call between two boys playing and their mother, who was on vacation in France. It was brief - the boys wanted to get back to playing with their cousins, not spend time on the phone chatting.

The brevity of that 1997 call troubles Prince William and Prince Harry to this day -- for their mother, Princess Diana, would die in a car crash that night.

"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 says in a new documentary. "But that phone call sticks in my mind, quite heavily."

Harry tells the filmmakers the final chat is something he will regret until the end of his days.

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"Looking back on it now, it's incredibly hard. I'll have to sort of deal with that for the rest of my life," Harry said. "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."

The ITV documentary "Diana, Our Mother: Her Life and Legacy" is airing Monday on British TV. Excerpts from the film, and new family photographs, were released Sunday.

The show is one of a series of tributes to Diana expected as the 20th anniversary of her death on August 31, 1997, approaches.

It is only in the last year that William and Harry have spoken openly in public about their feelings about the sudden loss of their mother. William - second-in-line for the British throne after his father Prince Charles - was only 15 at the time. Harry was only 12.

Earlier this year, William and Harry released a video promoting mental health, and said they didn't talk about Diana's death -- even to each other -- for years.

"I always thought to myself, what's the point in bringing up something that's only going to make you sad? It ain't gonna change it. It ain't gonna bring her back," Harry said.

The ITV documentary chronicles Diana's charitable works, including her historic outreach to AIDS victims and her campaign to ban land mines.

In 2016, Harry told CBS This Morning's Norah O'Donnell that he hoped his mother would be "incredibly proud" of the work he's done in creating the Invictus Games for veterans.

"I hope she'd be sitting up there having her own little party and looking down thinking what we've achieved because it's a massive team effort. What we've achieved is absolutely brilliant," he said.

William and Harry also stress their mother's fun-loving side, which they say the public generally didn't see.

"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," says Harry.

William tells a story that reveals the privileged life they led as children - one day, Diana surprised him by having three of the world's top models waiting for him when he got home from school.

"She organized when I came home from school to have Cindy Crawford, Christy Turlington and Naomi Campbell waiting at the top of the stairs. I was probably a 12 or 13-year-old boy who had posters of them on his wall," William said. "I went bright red and didn't know quite what to say. And sort of fumbled and I think pretty much fell down the stairs on the way up."

William says he frequently tells his children -- Prince George, 4, and Princess Charlotte, 2 -- about Diana so she can be a presence in her grandchildren's lives.

"She'd be a lovely grandmother. She'd absolutely love it, she'd love the children to bits," he said.