Prince William and Prince Harry have spoken of their confusion and "disbelief" after being told that their mother had died.
Princess Diana was killed in a car crash in Paris on August 31, 1997. Her two sons were holidaying in Balmoral with their father Prince Charles at the time.
And in a startling new documentary, Diana, 7 Days – which will air on BBC1 on Sunday evening – the boys speak with raw honesty about the tragedy, how they discovered the news and how it affected them.
Speaking about Prince Charles breaking the news to them, Harry reveals: "One of the hardest things for a parent to have to do is to tell your children that your other parent has died.
"How you deal with that, I don't know.
"But you know, he was there for us.
All I cared about was I'd lost my mother, and I didn't want to be where I was.
"He was the one out of two left and he tried to do his best and to make sure that we were protected and looked after.
"But, you know, he was going through the same grieving process as well."
William, who was 15 at the time, recalls his reaction upon hearing the awful news: “I remember just feeling completely numb, disorientated, dizzy.
“You feel very, very confused. And you keep asking yourself: ‘Why me?’ All the time: ‘Why, what have I done? Why has this happened to us?’”
And William's brother, who was just 12-years-old when they were informed of the accident, was equally dazed. So much so, that he says he couldn't process the information.
He remembers his initial reaction as: “Disbelief, refused to accept it."
And in the documentary, the royal brothers go into heartbreaking detail about the aftermath of their mother's death and how they struggled to cope.
On the day of Diana's funeral – September 6, 1997 – thousands gathered along the streets of London to watch the cortège meander from Kensington Palace to Westminster Abbey, while millions more watched on TV.
As a consequence, the young boys were forced to grieve in the public spotlight – and they admit that was a difficult pill to swallow.
Says William: “I was very touched by it but none of it sank in.
“All I cared about was I’d lost my mother, and I didn’t want to be where I was.
“When we go out and do things like that, in order not to completely and utterly break down, we have to put on a bit of a game face.
“And you have to be quite strong about it because otherwise you’re a walking mess.”
Describing the process of walking behind his mother's coffin as “one of the hardest things I’ve ever done,” William reveals that he hid behind his fringe as a "safely blanket".
But when the brothers returned to Kensington Palace at the end of that most difficult of days, there was no respite from the attention.
“People wanted to grab us, to touch us," explains William.
“They were shouting, wailing – literally wailing at us – throwing flowers and yelling, sobbing, breaking down. People fainted, collapsed.”
Adds Harry: “What we were doing, what was being asked of us, was verging on normal then.
“But now it’s like: ‘You did what?’”