Prince William has confessed he and Prince Harry felt they let their mother down because they failed to protect her.
The Duke of Cambridge made the emotional admission in a major BBC interview about the week that followed Princess Diana's death in a car crash in 1997.
He said he and his brother were finally ready to talk about their mother "because we feel we owe it to her". He added: "I think an element of it is feeling like we let her down when we were younger."
William, who was 15 at the time, continued: "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 interview will feature in a 90-minute BBC1 documentary to mark the 20th anniversary of Diana's death in Paris in August 1997.
The princes told of the moment they heard the news that their mother had died, and described how they felt as young boys following their mother's coffin at her funeral, a scene that moved the world.
Harry admitted he was shocked by the public's show of love for his mother.
"When she died there was such an outpour of emotion and love which was quite, which was shocking," he said.
"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."
Princess Diana was killed in a car crash in Paris in August 1997 while her children were holidaying at Balmoral with the Queen.
The interview was made public days after Diana's biographer, journalist Tina Brown, described how the young princes were left in a state of confusion immediately after their mother's death.
She said Harry, then 12, was so puzzled that the royal palaces continued running as normal that he had to ask Prince Charles, "Is it true that Mummy's dead?"
The BBC's programme - which has a working title of Diana and will air this summer - promises to tell 'the inside story of the tumultuous and unprecedented week' following
Diana's shocking death. It is the first time that William and Harry have spoken about the immediate aftermath in such detail.
They will also reflect on Diana's life and what she meant to them, opening up about their lasting grief.
They have been more public about the matter in recent months. William told GQ magazine this week that he has only recently come to terms with Diana's death.
"I still find it difficult to talk about now because at the time it was so raw," he said.
He also lamented the fact that his mother never had the chance to meet his wife Kate or his children.
The BBC film will include interviews with close friends, political figures and journalists.
Diana died on August 31, 1997, after a car crash in Paris that killed her lover Dodi Fayed and their chauffeur Henri Paul.
The documentary follows a controversial BBC royal drama which questioned whether Charles was Harry's real father and depicted Princess Diana as a ghost.
King Charles III also portrayed Kate as a ruthless schemer. The programme, originally a West End play, drew fierce criticism for being insensitive to the royals.
The Diana film was unveiled last night as part of 35 hours of BBC history, science, religion and factual programmes.
Factual commissioning controller Alison Kirkham said: "No subject should be taboo. We can't and won't shy away from ambitious, complicated programmes."