Harry Speaks Out About His Last Phone Call With Diana Before Her Tragic Death 

Prince William and Prince Harry have shared poignant new photographs from their personal collection of their late mother, Princess Diana, as they open up about her life and legacy in a new documentary.

The deeply personal photos come from albums that the princes have been looking over in memory of the mother as part of ITV and HBO’s Our Mother Diana: Her Life and Legacy special, which airs Monday. August 31 will mark the 20th anniversary of Diana’s death in a Paris car crash.

The documentary features appearances by Diana’s brother Charles Spencer, Elton John and Rihanna, who took an HIV test alongside Harry in Barbados last December to help break the stigma of getting tested. Diana was an early and very public campaigner for HIV and AIDS awareness and treatment.

Prince William and Princess Diana in a photo from the princes’ personal collection

The princes, who say that they will not speak out at length about her like this again, also talk movingly about the fact that their last phone call from her – on the day she died – was so short. Being such young boys, aged 15 and 12 respectively, they were keen to get to back to playing with their cousins at the Queen’s Scottish retreat, Balmoral. “And if I’d known that that was the last time I was going to speak to my mother the things that I would – the things I would have said to her,” Harry says in the documentary.

While discussing the documentary with reporters — including PEOPLE — at a meeting in Kensington Palace, William said, “This is a tribute from her sons to her.” He added that standing before a group about to watch the documentary was “possibly the most nervous I’ve ever been.”

Prince Harry and Princess Diana in a photo from the princes’ personal collection

“It’s one thing doing a documentary like this and it’s another thing standing front of all of you guys and explaining about something very personal to you,” he said. “Nevertheless, I think it’s very important that I’m here to do that.”

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And William – who like Harry finds talking about his mother so intensely and publicly difficult — vowed, “We won’t speak as openly and as publicly about her again.”

He added, “We feel hopefully this film will provide the other side from her close family friends that you might not have heard from 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 humor, and what she was like as a mother, which probably hasn’t come across before in many other pieces from other people.”

Prince William and Prince Harry in a photo from the princes’ personal collection

Harry also revealed he has only cried tears of grief twice – at her graveside at Althorp on September 6, 1997, and since then “maybe, maybe once. ”

“So there’s, you know, there’s a lot of — there’s a lot of grief that still needs to be let out.”

Harry 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.”

RELATED VIDEO: William Reveals His Mom’s Supermodel Surprise in a Sneak Peek from Diana, Our Mother: Her Life and Legacy 

Diana, Our Mother: Her Life and Legacy will air Monday at  10 p.m. on HBO in the U.S. and 9 p.m. in the U.K. on ITV.

The show is one of several commemorations of the Princess in print and on film. PEOPLE and ABC have partnered in a two-night television event The Story of Diana, airing on Aug. 9 and 10 at 9 p.m. ET.

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