The royal, who lost his mum when he was only 15 years old, leads a group of famous personalities who are trying to bring awareness to the cause with the video campaign.
The heartbreaking advert starts with former Bake Off judge Mary, who reveals what she would say to her son William if she had one more minute with him.
Visibly emotional, Mary Berry says with a cracking voice that she would thank William, who died when he was 19 after being involved in a car crash, for 'being a brilliant son'.
Rio Ferdinand is also featured in the campaign, talking about what he would say to his late wife Rebecca, who died of cancer in 2015.
'If I was able to have one more minute with Rebecca, I'd probably, well I'd definitely, ask her about what she wanted with the children growing up at different stages, poignant moments in their development', the former footballer said before adding that he would tell her he loves her.
Ben Fogle and his wife Marina, meanwhile, open up about the pain they felt when their baby was stillborn.
'I suppose the hard thing is that we didn't even have the first minute, to say anything to him, that's probably the answer', Ben said.
Marina added: 'We would say what we tell our children every night and every morning, that we love them.'
The campaign, which aims to raise awareness for the cause, also includes award-winning actor Jason Watkins, who lost his daughter to sepsis in 2011.
He said of his daughter Maude, who passed away when she was two: 'I would remind her of all the wonderful things she gave us and all the wonderful things she did.
'I would tell her that she will never be forgotten and that she's handed on all her beauty to her sister and to us.'
The campaign also features children and families talking about the loved ones they've lost, and is appealing for donations to the charity who supports bereaved families and helps them rebuild their lives.
The charity's chief executive, Ann Chalmers, told the Manchester Evening News: 'The film touches on the longing that many bereaved families tell us they have, to be able to have more time with the person who has died.
'In our 23 years of experience of supporting children, young people and families, this is something that is commonly expressed and that many people will relate to.
'Child Bereavement UK deals with a subject that many people find too unbearable to contemplate, the untimely deaths of parents and children, but every year, child bereavement is experienced by thousands of families in the UK.'