Prince William has opened up about the loss of his mother, Princess Diana, saying he felt "pain like no other" after her death in a car accident in 1997.

Speaking in a BBC documentary about mental health, the Duke of Cambridge said British people -- particularly men -- should feel comfortable talking about mental health issues.

"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, and you know that in your life it's going to be very difficult to come across something that's going to be even worse pain than that," William said.

"Particularly in Britain as well, we are nervous about our emotions," he added. "We're a bit embarrassed at times. You know, the British 'stiff upper lip' thing -- that's great, and we need to have that occasionally, when times are really hard there has to be a moment for that.

"But otherwise, we've got to relax a little bit, and be able to talk about our emotions, because we're not robots."

The Duke called for greater openness on emotional well-being while speaking with soccer players Peter Crouch, Danny Rose, Thierry Henry, Jermaine Jenas, and England manager Gareth Southgate, as part of the BBC's "Royal Team Talk" film on mental health.

He was 15 when his mother, Diana, Princess of Wales, was killed in a car crash in Paris while her driver fled pursuing paparazzi.

In recent years, both William and his brother, Prince Harry, have highlighted a number of mental health causes. He set up a website in September aimed at improving mental health in workplaces, and in February he criticized professional football clubs in England, accusing them of failing to take the mental health of their players seriously.