William, 35, speaks candidly about his late mother in the upcoming ITV and HBO documentary, Diana, Our Mother: Her Life and Legacy, in which he reveals he felt Diana’s presence during the 2011 wedding.
“When it came to the wedding, I did really feel that she was there,” he says. “You know, there was times when you look to someone or something for strength and I very much felt she was there for me.”
Along with William, 32-year-old Prince Harry opens up about his late mother in the special which airs ahead of the 20th anniversary of Diana’s death in a Paris car crash. The royals have expressed how difficult it is to talk about the death, and William has even vowed, “We won’t speak as openly and as publicly about her again.”
“It’s one thing doing a documentary like this and it’s another thing standing front of all of you guys and explaining about something very personal to you,” he said during a recent discussion with reporters — including PEOPLE — at Kensington Palace.
“Nevertheless, I think it’s very important that I’m here to do that.”
The princes, who were 15 and 12 respectively when Diana died, also expressed their regret over their last conversations with their mother.
“If I’d known that that was the last time I was going to speak to my mother the things that I would – the things I would have said to her,” Harry says in the documentary. The brothers last spoke to Diana in a telephone call while they stayed in Balmoral with their grandmother, Queen Elizabeth II, and father Prince Charles.
Diana, Our Mother: Her Life and Legacy will air Monday, August 31, at 10 p.m. on HBO in the U.S. and 9 p.m. in the U.K. on ITV.
The show is one of several commemorations of the Princess in print and on film. PEOPLE and ABC have partnered in a two-night television event The Story of Diana, airing on Aug. 9 and 10 at 9 p.m. ET.