Prince William Recalls What It Felt Like To Walk Besides Princess Diana's Coffin 

Prince William has described how he felt the presence of his late mother Princess Diana as he and brother Prince Harry marched behind her coffin at her funeral.

The prince, who was just 15 at the time of his mother’s death almost 20 years ago, is speaking out in the new BBC documentary 

“It was one of the hardest things I’ve ever done, that walk,” William says. “It felt like she was almost walking along beside us to get us through it.”

In the documentary, Harry recounts meeting members of the public in the days between Diana’s death in Paris and funeral at London’s Westminster Abbey on September 6, 1997. “People’s hands were wet because of the tears they had just wiped away,” he says in the new trailer.

Earl Spencer, called the walk a “tunnel of grief” and told PEOPLE and

on ABC he didn’t believe that “tiny” Harry should have made the grueling walk.

“I was just so worried — what a trauma for a little chap to walk behind his mum’s body,” Spencer said. “It’s just awful. And, actually, I tried to stop that happening, to be honest.

“You could walk for 300 paces through silence and then somebody would start wailing and people would start really sobbing and crying out really lovely things to William and Harry.”

RELATED VIDEO: Prince William on Following Princess Diana’s Coffin: ‘It Felt Like She Was Walking Beside Us’

Spencer is also appearing in the BBC documentary, made by Henry Singer, director of

. And giving a rare interview is Diana’s eldest sibling, Lady Sarah McCorquodale. In it, she sums up the astonishing tumult around the world that followed Diana’s August 31 death. “My sister’s dying has provoked this national, international reaction.”

Also included in the documentary is former British Prime Minister Tony Blair.

1 This Is How You’ll Know When Prince Harry and Meghan Markle Are Engaged

2 Prince Charles and Prince William Attended the Royal Edinburgh Military Tattoo

4 When Prince Vincent Can’t Stop Crying on His First Day of School, His Royal Mom Has the Magic Touch

5 Meghan Markle Praises Her ‘Free Spirit’ Mom — and Names This Oscar Winner as Her Acting Inspiration