The Duke of Cambridge and Prince Harry both expressed regret for their last chat with their mum, which they cut short because they were too preoccupied with their cousins.

“All I do remember is probably regretting for the rest of my life how short the phone call was.” Prince Harry, who was 12 when Princess Diana died, said.

Her sons talk candidly of their pain, mourning, and dealing with the trauma of her premature demise, at the age of 36, 20 years ago, in a documentary about Diana, Princess of Wales.

Her “fun” parenting was also discussed by the princes.

They said Diana encouraged them to be “naughty” and smuggled candy to them.

She was a “total kid through and through” according to the princes, who knew “real life outside of palace walls”

They also recall “the best mother ever,” a mother who “brought a breath of fresh air to everything she did,” according to Harry.

The show includes never-before-seen pictures of the princes with their mother.

When Diana died, William, who was 15 at the time, recalls hearing the news as if a “earthquake had run through the house.” His brother admits that he has only cried twice since her death, the first at her funeral on the island of Althorp, where she is buried, and “probably only once since then. So there’s a lot of grief that still needs to be let out .”

Prince Harry and Prince William are seen browsing through Diana’s personal album as they discuss how their perceptions of their mother as an infant contrasted with her national reputation and presence as an activist for the homeless, Aids refugees, and the abolition of landmines.

Diana was killed in a traffic accident in Paris on August 31, 1997. William explains how the brothers were enjoying a “very good time” with their cousins at Balmoral, the Queen’s private Scottish estate, on the day she died.

“Harry and I were in a desperate rush to say ‘goodbye’, ‘see you later’ and ‘we’re going to go off’ … if I’d known what would happen I wouldn’t have been so blasé about it. But that phone calls sticks in my mind quite heavily,” he says.

Participating in the show was originally “quite daunting” but it turned out to be “a healing process as well” according to Prince William.

“We feel this is an appropriate way of doing that,” he said, adding that they wanted “her legacy to live on in our work.”

Harry admits that as a child, he believed that “not having a mother was normal.” He recalls becoming perplexed by the outpouring of sorrow, wondering, “How can so many people who have never met this woman, my mother, be crying and showing more emotion than I was feeling?”

The documentary captures their impressions of her as a fun mother in happier moments.

William recalls how she once surprised him by inviting three models to his home. “She organised 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. 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. I was completely and utterly awestruck .” “That was a very funny memory that’s lived with me forever.”

The last talk with their mother, the Duke of Cambridge said, weighed “quite heavily” on his conscience.

The incident occurred when the brothers were enjoying a “very good time” with their relatives at Balmoral, the Queen’s Scottish estate.

In the interview, Prince William says he remembers what his mother said, but he doesn’t go into depth about the discussion.

“It was her speaking from Paris,” Prince Harry said. “I can’t remember what I said, but all I remember is probably regretting how short the phone call was for the rest of my life.”