Prince William has said the "shock" of his mother's death is still with him, 20 years after she was killed in a car crash in Paris.
William, who was 15 when Diana, Princess of Wales died, said the loss of a parent "never leaves you".
"I still have shock within me - people say it can't last that long but it does," he told the BBC One documentary, Mind Over Marathon, airing on Thursday.
Prince Harry also said it was "only right" to talk about painful memories.
William, Duke of Cambridge, told the BBC that he had to "learn to deal with" the loss of a parent.
"The shock is the biggest thing [which] I still feel 20 years later, about my mother," William said.
"You never get over it, it's such an unbelievably big moment in your life that it never leaves you. You just learn to deal with it."
The two princes, along with the Duchess of Cambridge, are promoting the Heads Together mental health campaign, the London Marathon's charity of the year.
Prince Harry told BBC News that he had "shared just as much as everybody else" during the course of the campaign.
He said: "Based around what people have experienced - and the mental anguish that's happened - it was only right to share my experiences".
He said he hoped that talking about the loss of his mother would "encourage others to come forward".
During the weekend, Harry revealed that he had received counselling to cope with the impact of Diana's fatal car crash in August 1997.
He told the Daily Telegraph that it was not until his late 20s that he processed the grief - after two years of "total chaos" and coming close to a "complete breakdown".
In the two-part documentary, the princes and Kate meet a group of 10 runners living with or affected by different mental health issues, as they prepare for the start of the marathon on Sunday.
The two-part series, Mind Over Marathon, starts on BBC One at 21:00 BST on Thursday 20 April, as part of a Minds Matter series of programming about mental health issues.