Watch: Princess Diana Funeral Pallbearers Recall the Heart-Stopping Moment They Nearly Dropped Her Coffin

The 20th anniversary of Princess Diana's death will be marked Sunday with two new documentaries on the Smithsonian Channel. One of the specials, Diana: The Day We Said Goodbye, focuses entirely on the late royal's funeral, which was seen by 2.5 billion people across the world in September 1997.

The documentary, narrated by Academy Award–winning actor Kate Winslet, features rare interviews with those who attended or took part in the funeral ceremony at Westminster Abbey, including two of the pallbearers who carried the Princess of Wales's coffin into the church.

In an exclusive clip from The Day We Said Goodbye, chief pallbearer Richard Williams, captain of the Welsh Guards at the time, speaks about how he and his colleagues avoided near disaster when the slippery floor of the abbey caused them to step out of sync and nearly fall.

"I can remember the squeak of the guys' boots on the floor, and I can also remember very clearly hearing that they started to get a little bit out of sync, and then of course it starts swaying," says Williams in the documentary.

"We had studs on our boots and also metal plates on them. Metal and marble doesn't go. It was really slippery," adds another pallbearer, Philip Bartlett.

Hilary Hares of the charity Aspire was sitting in the congregation as Diana's coffin was brought into the abbey.

She recalls: "The pallbearers were all incredibly young, and I also remember that one of them stumbled as they came along our side of the abbey. There was almost a perceptible intake of breath from the people on our side."

Bartlett recalls the moment, adding: "Started going under the archway, then one of the boys slipped."

"My heart stopped for a moment, thinking this would just be the most awful thing," Hares says.

The pallbearers managed to reposition themselves and completed the long walk with Diana's coffin without incident.

"I don't think we would have ever dropped it, 'cause you always had in the back of your mind who you were doing it for. You're doing it for the public, and you're doing it for the boys," says Bartlett, referring to Princes William and Harry.

August 31 marks 20 years since the Princess of Wales died from injuries sustained in a car crash in Paris.

Diana: The Day We Said Goodbye premieres on Smithsonian Channel, Sunday at 9 p.m.