PRINCES William and Harry said Diana, Princess of Wales, had "always been there" for them as they spoke openly about their mother's death in their first interview for US television

The princes said the last decade had passed "really, really slowly", and Prince Harry added that he will "never stop wondering" about what happened in the Pont de l'Alma tunnel in Paris, France, on August 31, 1997.

It is believed to be the first time the princes have spoken so openly about their mother's death.

The full interview will be screened on NBC News on Monday.

In extracts released today, Prince Harry said: "Over the last ten years I personally feel as though she has been... she's always there."

He said the time had passed "really, really, slowly".

"She's always being a constant reminder to both of us and everybody else.

"And therefore I think when you're being reminded about it, it does take a lot longer and it's a lot slower."

He added: "It's weird because I think when she passed away there was never that time, there was never that sort of lull. There was never that sort of peace and quiet for any of us – the fact that her face was always splattered on the paper the whole time."

He went on: "You know, when people think about it, they think about her death. They think about how wrong it was. They think about whatever happened.

"I don't know for... for me personally whatever happened that night. Whatever happened in that tunnel. You know no one will ever know. And I'm sure people will always think about that the whole time."

Asked if he had stopped wondering, the prince said: "I'll never stop wondering about that."

His older brother added: "Just for my personal opinion, when you knew somebody or someone that important to you, you always think about, you know.

"I mean, so after it happened we were always thinking about it. And there's not a day goes by I don't think about it once in the day. And so for us is a very slow and it's a lot – it has been a long time."

Last week, a documentary featuring photographs of Diana as she lay fatally injured following the car crash was broadcast in the UK, despite pleas from the royal brothers.

The princes had called for the removal of the images from the film Diana: The Witnesses in the Tunnel because plans to screen the pictures left them "deeply distressed".

But Channel 4 went ahead with the use of the controversial photographs in the documentary.

William and Harry are known to have been frustrated over the years by the persistent conspiracy theories surrounding the Princess's death.

Clarence House said Harry's comment about how he would never stop wondering referred to whether the crash could have been prevented, and was not about speculation over the cause.

A spokesman for the princes said: "Of course Prince Harry and Prince William will always be thinking a lot about what happened that night and whether the accident could have been avoided.

"But they have never doubted their mother's death was an accident.

"When the Stevens report came out last December, they said they hoped its conclusive findings would put an end to the speculation."

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