The Duke of Cambridge and Prince Harry have said they wanted to make a documentary about their mother because they felt they had let her down by not protecting her.
BBC documentary Diana (a working title) will tell "the inside story of the tumultuous and unprecedented week that followed the death of Diana, Princess of Wales".
The film features in-depth interviews with her sons, speaking in detail about that week's events from the moment they heard the news of their mother's death to the day of the funeral.
In the documentary, Prince William said: "Part of the reason why Harry and I want to do this is because we feel we owe it to her.
"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."
They will also reflect on Diana's life and what she meant to them, both then and now.
Prince Harry said: "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.
The 90-minute BBC1 documentary comes after the broadcaster aired controversial royal drama King Charles III, adapted from a stage play, in which the Prince of Wales succeeds his mother but sparks a constitutional crisis.
And it has been revealed days after the BBC announced a new drama, in which the impact of Diana's death on the lives of ordinary people is explored.
The documentary was announced as part of 35 hours of new commissions across history, science, religion, documentaries and factual entertainment on the BBC.
Alison Kirkham, controller of factual commissioning, said: "We are living in a period of seismic change when it feels harder than ever to get to grips with what is happening around us.
"In an era of false facts and fake news, it is the role of a proudly independent BBC to respond by offering a trusted lens through which to view and understand the world.
"Just a few years ago, many within the industry were predicting the demise of factual (programmes) in a multi-channel, multi-choice world.
"In fact the opposite has proven to be true.
"No subject should be taboo. We can't and won't shy away from ambitious, complicated programmes."