Celebrities shied away from supporting the Royals' Heads Together charity because they feared the stigma surrounding mental health, Prince William has said.
The Duke of Cambridge revealed that "not one person wanted to be involved" when he, the Duchess of Cambridge and Prince Harry were recruiting support for their flagship campaign three years ago.
Heads Together, which focuses on fighting prejudice associated with mental health and enabling people to access help, has since achieved widespread recognition and attracted the support of numerous celebrities, including Stephen Fry, Rio Ferdinand, Andrew Flintoff and Ruby Wax.
However, the Duke said that it was only after the royal trio put their "necks on the line" that they began to secure celebrity involvement.
Addressing the World Economic Forum at Davos, he said: "What was very interesting when we set up the campaign was that not one celebrity wanted to join us, not one person wanted to be involved in the mental health campaign Heads Together.
"We reached out to a lot of people and no one, when we started, was interested in being part of Heads Together, because it was mental health."
The initiative began breaking into the public consciousness in 2017, the year it was designated lead charity for the London Marathon and in which Prince Harry revealed he had sought counselling for mental health problems.
The Duke of Cambridge has also spoken of seeking help after witnessing traumatic incidents in his roles as an air ambulance pilot.
"Obviously once we started getting the ball rolling, once we started showing people a lot more what we were going to do, people realised Catherine, Harry and I had actually put our necks on the line here, that actually it was okay," he said.
"Then some very brave people came forward, from celebrities and normal people who decided this was really important. They bravely took on the talk of speaking out about mental health."
The Duke was speaking alongside Jacinda Ardern, Prime Minister of New Zealand, which has a high youth suicide rate, as well as the CEO of HSBC John Flint, who is driving an initiative to improve the mental wellbeing of the bank's 233,000 global employees.
Improvements to mental health provision, including a possible four-week minimum treatment target for children and young people, formed a major plank of the NHS long term plan announced earlier this month, following years of negative headlines about a "Cinderella service".
However, the Duke said society as a whole needs to become better at allowing encouraging people to talk about mental health.
He said the British were particularly embarrassed about discussing emotions and that the Second World War had entrenched a culture of "not talking" which has continued down the generations.
"It [the war] was so destructive and devastating that I think a whole generation just decided that this was the best way of dealing with it," he said.
"They then, completely by accident, passed this on to the next generation. We all want to learn from our parents, we want to learn from how they deal with things. So a whole generation then inherited that this was the way you deal with problems, you just don't talk about them.
"I think now there's a generation here that is finally realising that this is not normal, that we should talk about it."
Heads Together currently works with young people, emergency services, homeless and veterans charities.
Other celebrity endorsers have include Alistair Campbell, the newsreader Mark Austin and the rapper Stephen Manderson, known as Professor Green.
• Lifeline: 0800 543 354 (available 24/7)
• Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• Youth services: (06) 3555 906
• Youthline: 0800 376 633
• Kidsline: 0800 543 754 (available 24/7)
• Whatsup: 0800 942 8787 (1pm to 11pm)
• The Word
• Depression helpline: 0800 111 757 (available 24/7)
• Rainbow Youth: (09) 376 4155
• CASPER Suicide Prevention
If it is an emergency and you feel like you or someone else is at risk, call 111.