Britain's Princes William and Harry have spoken of their regret over a rushed final phone call with their mother, Princess Diana, on the night she died.

In interviews described as the most personal they have ever given about their mother, Prince William says he and his brother were in a "desperate rush to say goodbye".

"If I'd known now obviously what was going to happen I wouldn't have been so blase about it," he says.

Harry adds that he will probably regret for the rest of his life "how short that phone call was".

He says the discussion, which will be aired in a programme on ITV on Monday night, was the first time the brothers had ever spoken about Diana "as a mother".

Image: YouTube/HBODocs

"Arguably, probably a little bit too raw up until this point," Harry says. "It's still raw."

Prince William describes sharing memories of his mother as "cathartic" and "quite a healing process".

"There's not many days that go by that I don't think of her," he says. "Her 20th anniversary year seems a good time to remember good things about her and maybe provide a different side to her people haven't seen before."

Footage released ahead of the broadcast shows the two brothers looking through a photograph album which was put together by Diana.

"A total kid through and through"

William and Harry have released three of their favourite photos taken from the album, including one where Diana is holding Prince William at the time she was pregnant with Prince Harry.

Studying the photo, William says: "Believe it or not, you and I are both in this photograph, you're in the tummy."

Her sons talk at length about her sense of humour, including a moment when she invited supermodels Cindy Crawford, Christy Turlington and Naomi Campbell to meet Prince William at Kensington Palace.

"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," he said.

"I think I pretty much fell down the stairs on the way up. I was completely and utterly awestruck."

Prince Harry describes Princess Diana as "a total kid through and through" whose motto was "you can be as naughty as you want, just don't get caught".

"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," William says.

He explained the accounts from the film would be 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".