After years of speculation, Prince William has finally revealed who decided that he and Prince Harry should walk behind their mother's coffin at her highly publicized funeral.

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In a new documentary for BBC, "Diana, 7 Days," Prince William said it was his grandfather, Prince Philip, who suggested the boys follow behind their mother's casket, People reported.

“It wasn’t an easy decision, and it was a collective family decision to do that. It was one of the hardest things I’ve ever done. But we were overwhelmed by how many people turned out; it was just incredible. There was that balance between duty and family, and that’s what we had to do,” William said.

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Harry added, “I think it was a group decision. But before I knew it, I found myself with a suit on with a black tie and a white shirt, I think, and I was part of it. Genuinely, I don’t have an opinion on whether that was right or wrong. I am glad I was part of it. Looking back on it now, I am very glad I was part of it.”

William said it was a very "long and lonely walk," even though he was walking with his brother, their father, grandfather and Diana's brother Charles, the Ninth Earl Spencer, who recently said he opposed the boys walking in the procession.

“Both our parents brought us up to understand that there is this element of duty and responsibility, that you have to do things you don’t want to do,” William said. “When it becomes that personal, walking behind your mother’s funeral cortege, it goes to another level of duty. But I just kept thinking about what she would want, and that she’d be proud of Harry and I, and effectively she was there with us. It felt like she was walking alongside us to get us through it.”

“Diana, 7 Days” airs on Sept. 1 on NBC.

Remembering Princess Diana