Prince Harry wants the phrase "mental health" replaced with "mental fitness"

Tessy kicked off the event by describing Harry as "a passionate advocate for mental health" and added: "He actually said to me, as we came in, that actually, we should no longer talk about mental health, we should talk about mental fitness.

"His dedication to this cause strengthens our mission."

Harry added mental health was a "very, very important subject that affects every single one of us from top to bottom, from the youngest to the oldest".

The Duke - who has son Archie, five, and daughter Lilibet, three, with hiw wife Meghan, Duchess of Sussex, also paid tribute to the way young winners of the Diana Award were sparking change in society.

He added to the crowd about how politicians needed to step in to stop the mental health crisis: "Surely one of the solutions here is for governments to implement, or at least find people-young people like yourselves-and bring them into decision making, policy-making situations, before the problems exist.

"We're very good at creating problems for ourselves to try and solve, but surely by bringing young people in at the early stages... surely that is where the difference is going to be made.

"Why is that not happening?"

He added in reference to his late mother Princess Diana, who was killed aged 36 in 1997 in a Paris car smash: "I know that my mum would be incredibly proud of you guys... the way that you do it, your activism, your compassion, those two things are so true to how my mum led her life and what she believed in. And the way that you do it is incredible, so thank you.

"Every single one of us needs courage in order to really move the dial and create positive change in today's world, probably more so now than ever.

"Now we need to listen, constantly listen, and then act on the advice and the vision that you have because otherwise it's your future that's being stolen from you and that is unacceptable."

Harry and his older brother Prince William, 42, have spent years supporting the Diana Award, which was set up in their mother's memory to reflect her belief young people had the power to change the world.