Diana, Our Mother: Her Life and Legacy tells the personal story of the princes who were just 15 and 12 when she died in a car crash in Paris on 31 August 1997.
And the undeniably intimate glimpse of life with the ‘Queen of Hearts’ will not be repeated – with Prince William, 35, speaking ahead of the screening to say: “We won’t be doing this again – we won’t speak as openly or publicly about her again, because we feel hopefully this film will provide the other side from close family friends you might not have heard before, from those who knew her best and from those who want to protect her memory, and want to remind people of the person that she was.
“The warmth, the humour and what she was like as a mother, which probably hasn’t come across before in many other pieces and from other people.”
Here are five revelations from the show:
Like mother like son
Diana’s friend Sir Elton John appeared with the princes to talk about his bond with their mother and how her charity work made her a trailblazer in the royal family.
Sir Elton said: “She had this incredible ability, which he [Prince Harry] kind of inherited – and I’ve told him that and he said thanks very much – to make people feel at ease and them feel that everything’s going to be alright.
“I haven’t experienced may people in my life who’ve had that ability. She could walk into a room full of people and make them feel as if everything was great.”
Speaking about Diana’s work with Aids patients, Elton said: “It was considered to be a gay disease and for someone who was within the royal family and who was a woman and who was straight and to have someone care from the other side was an incredible gift.”
When asked how he keeps the memory of his mother alive for his own children, the Duke of Cambridge said: “I think, constantly talking about granny Diana, so we’ve got more photos up round the house now of her and we talk about her and stuff.
“It’s hard because obviously Catherine didn’t know her, she cannot really provide that level of detail.”
William said he regularly speaks about his mother when he puts his children Charlotte and George to bed, explaining: “there are two grandmothers in their lives, so it’s important they know who she was and that she existed.”
He joked: “She’d love the children to bits, but she’d be a nightmare. She’d come, probably at bath time, cause an amazing scene, bubbles everywhere, bath water all over the place, and then leave.”
‘She was one of the naughtiest parents’
Princess Diana may have been battling with depression and marriage problems, but she maintained a joyful atmosphere with her boys.
“She was one of the naughtiest parents,” remembered Harry, 32, with William adding: “She was very informal and really enjoyed the laughter and fun. But she understood that there was a real life outside of Palace walls.
Looking back on photographs of themselves, the brothers felt moved to remark on their mother’s “bizarre” sense of style.
Harry said: “One thing I would love to ask her now, because I genuinely think that she got satisfaction out of dressing myself and William up in the most bizarre outfits.
“Normally matching. It was weird shorts and 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?’”
The last phone call
The brothers reveal their heartache at cutting short their final chat with their mother, with Harry admitting it is something he will regret “for the rest of my life.”
The boys were at Balmoral and wanted to play with their cousins when she called.
William said: “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 blasé about it and everything else.
“But that phone call sticks in my mind, quite heavily.”
Harry recalled: “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.
“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.”
Children of divorce
Sadly, the princes had not seen their mother for almost a month when she died.
Harry said: “The two of us were bouncing between the two of them and we never saw our mother enough or we never saw our father [Prince Charles] enough.”
It was a combination of her charity work and her romance with Dodi Al-Fayed that kept her from her boys in the last weeks of her life.
In the documentary, Harry tells an anti-landmine campaigner: “You saw my mother more recently than I did.”
According to reports, Diana died the day before she was due to be reunited with her children.
Diana, Our Mother: Her Life and Legacy will air on ITV on Monday 24 July at 9pm.