Britain's Prince William and his brother Harry have spoken of their regret over the last conversation they had with their mother, saying the phone call was "desperately rushed".
In a new TV documentary timed to coincide with the 20th anniversary of Diana's car-crash death in Paris on August 31, 1997, the princes tell how they spoke to their mother shortly before she died.
"Harry and I were in a desperate rush to say goodbye, you know, 'see you later'," said William (35) of the call their mother made from Paris.
"If I'd known now what was going to happen, I wouldn't have been so blase about it and everything else."
Prince Harry (32) said: "I can't really remember what I said but ... I'll regret for the rest of my life how short the phone call was."
At the time of their mother's death, William was just 15 and Harry 12.
In the 90-minute ITV documentary, called Diana, Our Mother: Her Life And Legacy, the brothers give the most intimate insight yet into their childhood.
With the help of Diana's closest family and friends, William and Harry tell of their mother's efforts to give them a normal childhood and reveal her final letters and phone call and her love of pranks.
They share one of her photo albums - found earlier this year and containing pictures of the brothers as children - while William speaks of his mother's "presence" as a source of comfort before his 2011 wedding to Kate Middleton.
The film reveals how the divorce of Prince Charles and Diana left the brothers constantly travelling between houses, and of how their mother's death was like an "earthquake".
Introducing the film from Kensington Palace, William said he and Harry had never spoken so frankly in public before, explaining that the 20th anniversary of Diana's death felt like an "appropriate time to open up a bit more".
"We won't be doing this again," he said. "We won't speak as openly and publicly about her again.
"We feel that hopefully this film will provide the other side: from her closest family and friends - that you might not have heard before - from those who knew her best, and those who want to protect her memory and want to remind people of the person she was: the warmth, the humour, and what she was like as a mother.
"Harry and I feel very strongly that we want to celebrate her life, and this is a tribute from her sons to her."
Sitting with Harry to look at photos and talk about memories had been "cathartic", he said.
"We want her legacy to live on in our work, and we feel this is an appropriate way of doing that," William said.
The ITV film opens with the brothers leafing though the recently-found photo album.
One of the photos shows Harry on his first day of school, while another, taken on a beach holiday, shows him being hugged tightly by his mother.
"She would just engulf you and squeeze you as tight as possible," he says. "Being as short as I was then, there was no escape, you were there and you were there for as long as she wanted to hold you.
"Even talking about it, I can feel the hugs she used to give us and I miss that. I miss that feeling, I miss that part of a family, I miss having that mother to be able to give you those hugs and give you that compassion that I think everybody needs."
Before the trauma of their mother's death, William and Harry endured the fall-out from their parents' divorce, finalised in 1996 after a long and public battle.
"The two of us were bouncing between the two of them... we never saw our mother enough or we never saw our father enough," says Harry.
Exploring Diana's humanitarian work, from HIV awareness to homelessness, the film also reveals her final, incomplete challenge: landmines.
Harry tells how he found a "whole series" of letters around a month ago, dated the day of her death, and waiting for her signature.
"She knew exactly what needed to be done," says Harry. "And it's only recently that I've really understood the effect that she was having in those areas."
In the film, Harry speaks with two young victims of landmines in Bosnia, telling them they had seen his mother more recently than he had, as she had made the visit before going on holiday to Paris just a few weeks later.
In a light-hearted moment, Harry speaks with mock fury about the outfits he was compelled to wear as a child. He and William were photographed regularly in an array of elaborate and old-fashioned clothes.
"I genuinely think that she got satisfaction out of dressing [us] up in the most bizarre outfits," he says. "Normally matching. It was weird shorts and, like, little sorts of shiny shoes with the old clip on. I just think, 'how could you do that to us?'"
Eventually, the princes began to rebel, with William first refusing to match his brother and then Harry taking a stand.
"I like to think that she had great fun in dressing us up," says Harry. "I'm sure that wasn't it, but I sure as hell am going to dress my kids up the same way."
William and Harry tell how their mother tried valiantly to teach them about a normal life, despite their privileges.
"She made the decision that no matter what, despite all the difficulties of growing up in that limelight and on that stage, she was going to ensure that both of us had as normal a life as possible," says Harry.
"If that means taking us for a burger every now and then, or sneaking us into the cinema, or driving through the country lanes with the roof down of her old-school BMW listening to Enya... all of that was part of her being a mum."
Harry tells how Diana tried to embarrass him and William at every opportunity, from sending rude cards to them at school and roping in models to help her.
William tells how he once returned home, aged 12 or 13, to find Cindy Crawford, Christy Turlington and Naomi Campbell, the fashion models, waiting at the top of the stairs.
"I went red and didn't quite know what to say and sort of fumbled, and I think I pretty much fell down the stairs on the way up," he says. "I was completely and utterly sort of awestruck."
Diana's death, says William, was like an "earthquake". "There's not many days that go by that I don't think of her, you know - sometimes sad, sometimes very positively.
"You know, I have a smile every now and again when someone says something and I think, that's exactly what she would have said, or she would have enjoyed that comment.
"So they always live with you, people you lose like that. My mother lives with me every day."
For his part, Harry says: "There's not a day that William and I don't wish that she was still around.
"You know, and of course as a son I would say this, she was the best mum in the world."
Diana, Our Mother: Her Life And Legacy is on ITV tonight at 9pm