Prince William's heartbreak over late mum Princess Diana

Prince William has opened up about how difficult it was losing his mother, Princess Diana, in a new documentary aiming to tackle the stigma surrounding mental health.

Reflecting on the death of his mother, who tragically died in car crash when he was just 15, explained he felt "pain like no other pain".

But he also added it's helped him relate to people who have also suffered the same type of bereavement.

"I've thought about this a lot, and I'm trying to understand why I feel like I do, but I think when you are bereaved at a very young age, any time really, but particularly at a young age, I can resonate closely to that, you feel pain like no other pain," he says in an upcoming BBC documentary titled, A Royal Team Talk: Tackling Mental Health.

"And you know that in your life it’s going to be very difficult to come across something that’s going to be an even worse pain than that."

However, the royal also adds, "It also brings you close to all those other people out there who have been bereaved."

The new doco is set to air in the United Kingdom on Sunday, May 19. The Duke of Cambridge sits down with soccer stars Peter Crouch, Danny Rose, Thierry Henry and Jermaine Jenas, as well as England manager Gareth Southgate.

The six are set to share stories about the various issues they have faced throughout their careers surrounding their mental health.

In the documentary, William, 36, also opens up about the need to break down the typical "British stiff upper lip thing" with people needing "to relax a little bit and be able to talk about our emotions because we're not robots".