Prince William and Prince Harry have said they regret how "rushed" their last phone call with Diana was.
The princes have revealed in an ITV documentary that they were desperate to end the conversation with their mother as they wanted to get back to playing outside.
It was the last time they would speak to Diana, who was killed in a car crash in Paris several hours later.
"All I do remember is probably regretting for the rest of my life how short the phone call was," Prince Harry said, who was 12 when his mother died.
Prince William, who was 15 at the time, admitted that the memory of their final conversation still "stuck in his mind quite, quite heavily."
"At the time Harry and I were running around minding our own business, you know, playing with our cousins and having a very good time," he said. "And I think Harry and I were just in a desperate rush to say goodbye, you know, see you later and we're going to go off."
"If I'd known now obviously what was going to happen I wouldn't have been so blasé about it and everything else," he added.
"Looking back on [that phone call] now, it's incredibly hard," Prince Harry said in the special documentary, which was made to mark the 20th anniversary of his mother's death. The one-off documentary will be broadcast on Monday (24 July) on ITV.
"I have to sort of deal with that for the rest of my life. Not knowing that it was the last time I was going to speak to my mum, and how differently that conversation would have panned out if I'd had even the slightest inkling that her life was going to be taken that night," he said.
The princes have never openly spoken about their mother's death before but felt that the 20th anniversary of her passing was the right time to do so.
"Harry and I feel very strongly that we want to celebrate her life, and this is a tribute from her sons to her," Prince William said, adding that making the documentary had been a "cathartic" process.
"It's been at first quite daunting opening up so much to camera, but going through this process has been quite healing as well," he said.
Prince Harry told The Telegraph in April that he had sought counselling in recent years to help him cope with his mother's death after years of refusing to talk about the impact it had on his life.
"It has been hard and it will continue to be hard, there's not a day William and I don't wish that she was still around and we wonder what kind of mother she would be now, and what kind of a public role she would have and what a difference she would be making," he said in the documentary.