On the night of her death Princess Diana called her two sons for a chat.

At the time, 15-year-old William and 12-year-old Harry rushed through the conversation so they could get back to playing with their cousins in Balmoral.

They had no idea that would be the last time they would talk to their mum. Just hours later, Diana was killed in a car crash in the Pont de l’Alma road tunnel in Paris.

Now, 35 and 32, Prince William and Prince Harry have opened up about their regrets over that fleeting conversation from their childhood.

In a documentary called Diana, Our Mother: Her Life and Legacy (due to air on Channel 7’s Sunday Night program next week) Prince William admitted that conversation “still sticks in [his] mind” 20 years later.

She was many things to many people, but to her two boys, Lady Diana was just mum. Coming soon to #SN7.
PREVIEW: https://t.co/4ZEQpxGCiP pic.twitter.com/czeMp5L5tt

— sunday night (@sundaynighton7) July 15, 2017

“Harry and I were in a desperate rush to say goodbye, you know, ‘see you later’. If I’d known now obviously what was going to happen I wouldn’t have been so blasé about it and everything else. But that phone call sticks in my mind, quite heavily,” Prince William said.

Harry has also wrestled with guilt over the years.

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

“Looking back on it now it’s incredibly hard. I’ll have to sort of deal with that for the rest of my life. Not knowing that was the last time I was going to speak to my mum, how differently that conversation would have panned out if I’d had even the slightest inkling her life was going to be taken that night.”

The documentary is the first time Prince William and Prince Harry have sat down together and spoken about their mum on air.

Gradually, as time has gone by, Princess Diana’s sons have become more open about the loss of their mother and the effect this has had on their mental health.

Speaking at a barbecue last year for the royal family’s mental-health initiative Heads Together, Harry revealed he went a long time without speaking about his mother’s death.

“Everything can be okay, but I really regret not ever talking about it for the first 28 years of my life,” he said.

“I never talked about it.”

“It’s OK to suffer, but as long as you talk about it. It’s not weakness. Weakness is having a problem and not recognising it and not solving that problem.”

The Duke and Duchess of Cambridge and Prince Harry will attend this year’s @londonmarathon together for the first time to support runners taking part in their mental health campaign @heads_together, which is this year’s Charity of the Year for the Marathon. All 39,000 Virgin Money London Marathon runners will be given a special Heads Together headband which they can wear on race day to be part of the national movement to end the stigma around mental health once and for all. During the week leading up to the marathon Their Royal Highnesses will attend several events in support of #TeamHeadsTogether – Heads Together wants to make this year’s marathon the ‘mental health marathon’ that gets the country talking about mental health and Their Royal Highnesses will cheer on all runners at points along the route on Marathon day!

A post shared by Kensington Palace (@kensingtonroyal) on Apr 11, 2017 at 3:35am PDT

He also told People magazine there was a “big gaping hole” left when she died.

“All I want to do is make my mother incredibly proud. That’s all I’ve ever wanted to do,” he said.

In the documentary, Prince Harry also reveals he’s only cried twice in the two decades since his mother’s death.

“There’s a lot of grief that still needs to be let out,” he said.

“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 added.

The documentary marks 20 years since the passing of Princess Diana and it’ll also feature candid conversations with Sir Elton John, Diana’s brother, and even Rihanna.