Prince William and Prince Harry have said they helped make a documentary about their mother because they felt they had "let her down" in not being able to "protect her".

The documentary seeks to tell "the inside story of the tumultuous and unprecedented week that followed the death of Diana, Princess of Wales".

This summer will mark the 20th anniversary of Princess Diana's death in a car crash in Paris on August 31, 1997.

Prince William was 15 and Prince Harry was just 12 at the time of their mother's death.

The film features in depth interviews with the Duke of Cambridge and Prince Harry, in which they speak in detail about the week following their mother's death.

"Part of the reason why Harry and I want to do this is because we feel we owe it to her," Prince Williams says in the documentary.

"I think an element of it is feeling like we let her down when we were younger. We couldn't protect her.

"We feel we at least owe her 20 years on to stand up for her name and remind everybody of the character and person that she was.

"Do our duties as sons in protecting her."

The pair will also reflect on Diana's life and what she meant to them, both then and now.

In the documentary, Prince Harry reflects on the "outpouring of emotion" following Princess Diana's death: "When she died, there was such an outpour of emotion and love which was quite ... which was shocking.

"It was beautiful at the same time, and it was amazing, now looking back at it, it was amazing that our mother had such a huge effect on so many people.

"When you're that young and something like that happens to you, I think it's lodged in here, there, wherever - in your heart, in your head and it stays there for a very, very long time.

"I think it's never going to be easy for the two of us to talk about our mother, but 20 years on seems like a good time to remind people of the difference that she made not just to the royal family but also to the world."

The film will also include interviews with close friends, political figures and journalists, many of them speaking for the first time about the events of that week in August 1997.