The Duke of Cambridge has teamed up with pop superstar Lady Gaga to encourage people to open up about their feelings and bring an end to the "shame" of talking about mental health issues.
In a video watched by around 15,000 people live on Facebook, William and Lady Gaga chatted about how important speaking freely about mental health can be in helping to shatter the stigma around it.
The Duke said: "It's time that everyone speaks up and really feels very normal about mental health, it's the same as physical health. Everybody has mental health and we shouldn't be ashamed of it and just having a conversation with a friend or family member can really make such a difference."
William, who with his brother Harry and the Duchess of Cambridge have championed the Heads Together campaign, has has called for an end to the "stiff upper lip" culture, saying he wants Prince George and Princess Charlotte to be able to talk about their emotions.
It comes after Prime Minister Theresa May praised Prince Harry for his bravery in revealing that he sought counselling to come to terms with the death of his mother, Diana, Princess of Wales.
Speaking from Kensington Palace to Lady Gaga at her home in Hollywood, William told the singer many of the issues he had seen from his work with veterans, homelessness and addiction charities stemmed from mental health issues.
And he praised her bravery in revealing through an open letter last year that she lives with post-traumatic stress disorder.
Lady Gaga said her changing mental health had "changed my life" and she had been "very nervous at first" speaking about it.
She said: "For me, waking up every day and feeling sad and going on stage is something that is very hard to describe.
"There is a lot of shame attached to mental illness, you feel like something's wrong with you.
"In my life I go, 'oh my goodness, look at all these beautiful, wonderful things that I have, I should be so happy', but you can't help it if in the morning when you wake up you are so tired, you are so sad, you are so full of anxiety and the shakes that you can barely think.
"It was like saying, 'this is a part of me and that's OK'."
Lady Gaga said talking more openly about mental health would allow people dealing with it to feel like "we are not hiding any more".
She said: "Even though it was hard, (it was) the best thing that could come out of my mental illness was to share it with other people and let our generation, as well as other generations, know that if you are feeling not well in your mind that you're not alone and that people that you think would never have a problem, do.
"We have to make the strongest, most relentless attempt we can to normalise mental health issues, so that people feel like they can come forward."
William added: "It's OK to have this conversation, it's really important to have this conversation and that you won't be judged.
"It's so important to break open that fear and that taboo which is only going to lead to more problems down the line."
The pair also made plans to meet when Lady Gaga comes to the UK in October, and the singer said: "We have to make the strongest, most relentless attempt we can to normalise mental health issues so that people feel like they can come forward."
Both William and Harry have been increasingly vocal about the importance of openness around mental health.
The Duke has highlighted the importance of role models speaking out their feelings, applauding grime artist Stormzy for talking about suffering from depression, and has said he and Kate want Prince George and Princess Charlotte to feel comfortable about discussing their emotions.
He told charity publication CALMzine: "Over the past year we have visited a number of schools together where we have been amazed listening to children talk about some quite difficult subjects in a clear and emotionally articulate way, something most adults would struggle with.
"Seeing this has really given me hope things are changing and there is a generation coming up who find it normal to talk openly about emotions."
William praised Stormzy's revelation about his depression as "incredibly powerful", saying: "There may be a time and a place for the 'stiff upper lip', but not at the expense of your health."
Harry, who was 12 when his mother was killed in a car crash in Paris, has revealed that it too him until his late 20s to process his grief.
In an interview with the Daily Telegraph, Harry, 32, said he spent nearly 20 years "not thinking" about her death and eventually got help after two years of "total chaos".
He admitted shutting down his emotions after losing his mother had "a quite serious effect on not only my personal life but my work as well", and that he eventually sought help after his brother told him he needed to deal with his feelings.
Responding to his comments, Mrs May said: "Mental health problems affect people of all ages and all backgrounds.
"The bravery of those in public positions who speak out about their experiences helps smash the stigma around mental health and will help thousands of people to realise they are not alone.
"If we are to tackle this injustice, we must forge a new approach that recognises our responsibility to each other, and make mental illness an everyday concern for all of us and in every one of our institutions."
Ministers are thought to be considering putting NHS professionals in secondary schools full-time, under plans understood to be part of a green paper on young people and mental health which will be published later this year.
Heads Together, an umbrella organisation for mental health charities, is the London Marathon's charity of the year, and on Tuesday William will attend a screening of BBC documentary Mind Over Marathon, which follows a group of runners affected by mental health as they prepare for the race.
The Duke and Duchess and Harry, who hope the race will be known as "the mental health marathon", will hand out medals on the finish line in the Mall on Sunday.