In an emotional and heartbreaking promo for a new documentary about Princess Diana, her sons Prince William and Prince Harry remember her as "informal," "one of the naughtiest parents" and the "best mum in the world."

This August will mark the 20th anniversary of the day the Princess of Wales, then the world's most beloved royal, died in a car crash in Paris at age 36. William was 15 and Harry was 12 at the time of her death.

In the new documentary Diana, Our Mother: Her Life and Legacy, William, 35, and Harry, 32, look at childhood photos of them with their mother and reminisces about her.

"You and I are both in this photograph," William says, holding up a picture of a pregnant Diana holding him. "You're in the tummy."

Prince William, Prince Harry, Princess Diana

HBO; Getty Images

William said Diana "was very informal and really enjoyed the laughter and the fun."

"She was one of the naughtiest parents," Harry added, as footage of the two as kids having fun with their mother is shown.

William said Diana "understood that there was a real life outside of palace walls."

"She was our mum," Harry says. "She still is our mum. And of course, as a son I would say this, she was the best mum in the world."

"She smothered us with love, that's for sure," he added.

The documentary Diana, Our Mother: Her Life and Legacy is set to air on ITV in the U.K., CBC in Canada and on HBO later this year.

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