Prince William speaks on carrying burden of grief and shock at his mother's death for 20 years

The Duke of Cambridge has spoken out about the shock he still feels 20 years after his mother was killed in a car crash in Paris.

Prince William, who was 15 when Princess Diana died, said the loss of his mother was “such an unbelievably big moment”, adding: “the shock never leaves you”.

He spoke to Rhian Burke, whose one-year-old son George died suddenly from pneumonia in February 2012. Five days later, her husband took his own life.

They were being filmed as part of BBC documentary Mind Over Marathon, which explores mental health issues with London Marathon runners.

Ms Burke, 39, asked the Duke whether her other two children would eventually heal.

"You were obviously a little bit older than my children [when Diana passed away], but I obviously worry about them growing up, they'll be OK won't they?” she said.

William replied: “With a mum like you they'll be absolutely fine. That's true, they will be. Because you're aware of all this, you're already a step ahead. Once you start rationalising it, and understand, 'I'm really angry and really upset about something,' then you can, I think, relativise it and deal with it.

"You'll provide the blanket of stability and understanding they need. I can't tell you enough, you doing this is an incredibly big, positive step, and I really hope it brings you what you need.”

Elsewhere in the film, he said: "The shock is the biggest thing, and I still feel it 20 years later about my mother.

“People think shock can't last that long, but it does. It's such an unbelievably big moment in your life and it never leaves you, you just learn to deal with it.”

It comes after the Duke teamed up with Lady Gaga, who suffers from PTSD, in a bid to end the “shame” of speaking out about mental health. The pair were filmed having a FaceTime chat about how important it is for people to feel they can speak openly about mental illness.

At a private screening of the documentary yesterday, a clearly moved William said: “Wow. I am speechless, actually, and quite emotional. I'm going to take a moment to calm myself down. I really think this is a pivotal moment in the change to mental health, and that we are on the cusp of something really big.

“As you can see, I have my own reasons for being involved in mental health, after what happened to my mother when I was younger, but equally the charitable work that I do at the moment and the areas I am involved in, it all comes back to mental health.

“We need to make mental health normal, and we need to treat it in the same way we treat physical health. It has to be seen in the same way.”

Earlier this week, Prince Harry gave an emotional interview in which he revealed he sought professional help four years ago after struggling with the grief of Diana's death in August 1997, when William and Harry were just 15 and 12..

The BBC1 film airs on Thursday, and will see the Duke, Prince Harry and the Duchess of Cambridge follow 10 “unlikely runners” with different mental health issues training for the run.