"The Duke of Cambridge and Prince Harry have spoken of their enduring regret over their last conversation with their mother, disclosing they had been desperate to rush off the telephone and get back to playing instead. The Duke and Prince, who were 15 and 12 when Diana, Princess of Wales, died in a car crash, said they had been busy at Balmoral when she called them, and had no notion that the short phone call would be their last. The conversation, the Duke said, still weighs on his mind ""quite, quite heavily"", while Prince Harry admitted he would regret it for the rest of his life. The brothers spoke as part of a one-off documentary for television, in which they detail their warm memories of their fun-loving mother. Princes William and Harry reveal Princess Diana's naughty side in new ITV documentary 01:40 In their most extensive and honest television interview to date, the Duke and Prince speak fondly of their childhood with one of the world's most-photographed women, in the hopes of introducing the real her to a new generation.  The Duke, who now has two children of his own, discloses how he is keeping his mother's memory alive at home, while the Prince shares the weird and wonderful pranks the ""naughty"" princess loved to play on her sons. Speaking in an interview due to broadcast on ITV on Monday, the Duke described his last conversation with her, while she was in Paris and he was at Balmoral with his father, the Prince of Wales, and the wider Royal family. All I do remember is probably, you know, regretting for the rest of my life how short the phone call was ""At the time Harry and I were running around minding our own business, you know, playing with our cousins and having a very good time,"" he said. Prince Harry continued: ""As a kid I never enjoyed speaking to my parents on the phone. ""And we spent far too much time speaking on the phone rather than speaking to each other, because of just the way the situation [the divorce] was. ""And the phone rang and off he [William] went to go and speak to her sort of for five minutes."" The Duke said: ""And I think Harry and I were just in a desperate rush to say goodbye, you know, see you later and we're going to go off. ""If I'd known now obviously what was going to happen I wouldn't have been so blasé about it and everything else. ""But that phone call sticks in my mind quite, quite heavily."" Duke of Cambridge and Prince Harry Credit: The Duke of Cambridge & Prince Harry He told an on-screen interviewer that he did recall what she had said to him, but chose not to disclose it."" Prince Harry, who in April told the Telegraph he had sought counselling after years of refusing to talk about Diana's death, said he remembered being called to take his turn at the telephone receiver. ""It was her speaking from Paris. ""I can't really necessarily remember what I said, but all I do remember is probably, you know, regretting for the rest of my life how short the phone call was. ""And if I'd known that that was the last time I was going to speak to my mother the things I would have said to her."" He added: ""Looking back on it now, it's incredibly hard. I have to sort of deal with that for the rest of my life. ""Not knowing that that was the last time I was going to speak to my mum, and how differently that conversation would have panned out if I'd had even the slightest inkling that her life was going to be taken that night.""  Princess Diana's most iconic fashion moments The Duke of Cambridge and Prince Harry have spoken as never before about Diana, Princess of Wales, in an astonishing interview designed to teach a new generation about their mischievous mother. The brothers, now aged 35 and 32, have given the most intimate insight yet into life with their mother at Kensington Palace, as they open their family photo album for the nation. Diana, Princess of Wales with her sons, Harry and William, in 1992 Credit: Martin Keene/PA Wire In a 90-minute documentary, featuring her closest family and friends, the Duke and Prince will bring Diana's memory to life for those too young to remember her, detailing her efforts to give them a normal childhood, her final letters and phone call, and her love of pranks. They share her own home photograph album, found earlier this year and containing pictures of the princes as children, as the Duke speaks of how he felt her presence as a source of comfort before his 2011 wedding to Catherine Middleton. It will reveal how their parents' divorce left them constantly travelling between houses, her death was like an ""earthquake"", and how the Queen was at one point so concerned about her that she took friends aside. Detailing the extraordinary tricks Diana would play on them, including a memorable episode including supermodels at the top of the stairs, the Duke tells how he keeps her memory alive with his own young children, despite predicting she would have been an ""absolute nightmare"" grandmother. Introducing the film at a Kensington Palace screening last week, the Duke said he and his younger brother had never spoken so frankly in public before, explaining that the 20th anniversary of Diana's death in August this year felt like an ""appropriate time to open up a bit more"". Princess Diana holding Prince William while pregnant with Prince Harry Credit: The Duke of Cambridge & Prince Harry Saying he hoped the film would encapsulate the woman he would like the public to know, he said: ""We won't be doing this again. ""We won't speak as openly and publicly about her again, because 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 down with Prince Harry to look at photographs and talk about memories, he added, had been ""cathartic"", he said: ""It's been at first quite daunting opening up so much to camera, but going through this process has been quite healing as well."" As well as her sons, the film also features Diana's brother Earl Spencer, who speaks frankly about how the bitter divorce of their parents affected her, Sir Elton John, who campaigned with her and sang at her funeral, and a host of friends including William Van Straubenzee, Lady Carolyn Warren and Anne Beckwith-Smith. The Duke and Prince have also taken part in a BBC documentary, due out later this year and focusing specifically on the week following the Parisian car crash. They last month marked Diana's birthday by rededicating her grave at Althorp, the Spencer family home, and will commemorate the anniversary of her death in August. ""We want her legacy to live on in our work, and we feel this is an appropriate way of doing that,"" said the Duke. ""To remind not only the people who knew her, but also you have to remember this is 20 years ago now that she died and there are people who don't even know about her. ""We want to share the happiness and the warmth of her and what she was like as a person with a wider audience, and so came the documentary. ""I hope you enjoy it.""   The Family Album Princes William and Harry taking part in the documentary Credit: ITV The documentary opens with the Duke and Prince leafing though Diana's photograph album, only recently rediscovered at home and full of picture of them as children. Prince Harry, who stars in many of them, told William: ""Part of me never really wanted to look at them and part of me was waiting to find the right time where we could sit down and look at them together. One shows him on his first day of school, while another captures a beach holiday, where he is hugged tightly by Diana. She would just engulf you and squeeze you as tight as possible,"" he recalled, speaking to camera. ""And 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 now I can feel the hugs that 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."" The Queen's Worries The Queen, the documentary reveals, was so concerned about Diana in her low points that she took a friend aside quietly at Balmoral to check on her welfare. Harry Herbert, whose father was the 7th Earl of Carnarvon and racing manager to the Queen, said: ""I had a talk to the Queen about it at Balmoral. ""The Queen wanted to talk to me about it because she was so worried about Diana. ""After a lunch at Balmoral and going [on a walk] up high and looking down onto this beautiful setting of heather and Castle, and an incredibly important chat. A very personal chat. ""And the Queen wanted to know how was Diana feeling, and was it as bad as it was? ""It  was a sad discussion, a sad moment really because that was everything at its worst."" But he said, he had visited Diana at home in Kensington Palace when she was struggling, and even then her face would ""light up"" when her sons came ""thundering"" into her room. Divorce Before the trauma of Diana's death, Prince William and Prince Harry endured the fall-out from her divorce from the Prince of Wales, finalised in 1996 after a long and very public battle between their parents. ""There was the point of where our parents split and the two of us were bouncing between the two of them and we probably didn't...we never saw our mother enough or we never saw our father enough,"" Prince Harry said. ""You know there was a lot there was a lot of travelling and a lot of fights on the back seat with my brother, of which I would win. ""So there was all of that to contend with. And I don't pretend that we're the only people to have to deal with that. But it was, it was an interesting way of growing up."" Princes William and Harry: Our mother Diana was so loving 01:09 Diana's Legacy Exploring Diana's main causes, from HIV awareness to homelessness, the film also reveals her final, incomplete, challenge: landmines. Prince Harry tells how he found a ""whole series"" of letters, around a month ago; dated August 31 and waiting for her to sign them. ""She knew exactly what needed to be done,"" he youngest son said. ""She was writing letters to certain people to say right,, this is what needs to happen in order for this whole sort of tidal wave to change. ""And it's only recently over the years that I've actually really understood the effect that she was having in those areas and on an international scale as well."" In the film, he speaks with two young victims of landmines in Bosnia, telling them they had seen his mother more recently than he had. She had spent time with them after learning they had been injured by mines, going on holiday to Paris just a few weeks later while Prince Harry was at Balmoral. Their childhood outfits In a light-hearted moment, Prince Harry speaks with mock-fury about the outfits he was compelled to wear as a child, saying he would love to ask his mother why she chose them. The two young boys were regularly photographed in an array of elaborate and old-fashioned clothes, often matching. "" I genuinely think that she got satisfaction out of dressing myself and William up in the most bizarre outfits,"" he said. ""Normally matching. It was weird shorts and, like, little sort of shiny shoes with the old clip on. Looking back at the photos it just makes me laugh. ""I just think 'how could you do that to us'."" One by one, he said, the Princess began to rebel, with William first refusing to match his brother and then Harry taking a stand. ""So I like to think that she had great fun in dressing us up,"" he said. ""I'm sure that wasn't it, but I sure as hell am going to dress my kids up the same way."" A Normal Life Diana, her sons said, tried valiantly to teach them about a normal life, despite the privileges of their upbringing. ""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 life as possible,"" said Prince Harry. ""And 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 I think it was...All of that was part of her being a mum"". Diana, the Prankster If she strove for a normal life, Diana's love of pranks was anything but ordinary. Described as a ""total kid through and through"" by Prince Harry, the late princess', she attempted to embarrass her sons at every opportunity, from sending rude cards to them at school to roping in supermodels to help her. Prince William told how he once returned home, aged 12 or 13, to find pin-ups  Cindy Crawford, Christy Turlington and Naomi Campbell waiting for him at the top of the stairs. ""I went bright 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 said ""I was completely and utterly sort of awestruck. But that was a very funny memory. That's lived with me forever."" At other time, he said, she would post him ""the rudest cards you can imagine"" to boarding school, leaving him in fear of being spotted by a teacher. Prince Harry recalled how she would smuggle sweets into their socks when she came to watch them playing football, saying they would walk back to their tuck box with their clothes ""bulging"" with treats. If she worried about her sons following in her footsteps, it appears she did not show it. Prince Harry said: ""One of her mottos to me was: 'you can be as naughty as you want, just don't get caught'. Granny Diana If she excelled as a mother, Diana would have been an ""absolute nightmare"" as a grandmother, Prince William joked, as he discloses how he tries to keep her memory alive. Saying he is ""constantly"" mentioning ""Granny Diana"" at home, he has also mounted more photographs so that Prince George and Princess Charlotte learn about her. ""It's hard because obviously Catherine didn't know her, so she cannot really provide that level of detail,"" he said.  ""So I do regularly put George or Charlotte to bed, talk about her and just try and remind them that there are two grandmothers - there were two grandmothers - in their lives."" Asked how she would be like had she lived to enjoy the next stage of her family life, he added: ""She'd be a nightmare grandmother, absolute nightmare. She'd love the children to bits, but she'd be an absolute nightmare. ""She'd come and go and she'd come in probably at bath time, cause an amazing amount of scene, bubbles everywhere, bathwater all over the place and then leave. ""I want to make as much time and effort with Charlotte and George as I can because I realise that these early years particularly are crucial for children, and having seen, you know, what she did for us."" An earthquake Diana's death, Prince William said, was like an ""earthquake"", running through their lives with such shockwaves that it took a while to sink in. ""There's not many days that go by that I don't think of her, you know - sometimes sad, sometimes very positively,"" he said. ""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. And my mother lives with me every day."" Prince Harry acknowledge it ""has been hard and it will continue to be hard"", added: ""There's not a day that William and I don't wish that she was still around, and we wonder what kind of a mother she would be now, and what kind of a public role she would have, and what a difference she would be making. ""You know, and of course as a son I would say this, she was the best mum in the world."" The programme, 'Diana, Our Mother: Her Life and Legacy, will be broadcast on on Monday at 9pm on ITV."
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