"We went out to a lot of people and nobody, before we started, was interested in being a part of 'Heads Together' – because it was mental health," he said, speaking on a panel at the World Economic Forum in the Swiss resort of Davos.

William, who is second in line to the British throne, also opened up about his experiences of dealing with trauma during his daily work in the air ambulance, which he retired from two years ago.

Remembering a particularly affecting incident, he said: “I know if I hadn’t taken the action I did then, I would have definitely gone down a slippery slope and I would have been dealing with mental illness on a different level."

His fellow panelist, New Zealand's Prime Minister Jacinda Ardern, praised the prince for speaking out about mental health issues.

Read also: Prince William turns interviewer in Davos appearance with Attenborough

Ardern, at 38 the world’s youngest female head of government, has said she will introduce New Zealand's first ‘well-being budget’ to gauge how policy impacts the quality of people’s lives.

"I think unfortunately one of the sad facts for New Zealand is that everyone knows someone who has taken their own life. We're a small country, less than 5 million people, but last year more than 600 people committed suicide," she said.

"I’ve lost friends and I wouldn't have to look far in my cabinet to find others who have as well," Ardern added.

William, who has previously warned British men of the dangers of keeping a traditional "stiff upper lip" at times of stress, said he thought his country’s difficulties with expressing emotions came from a wartime generation who passed on a tendency not to talk about problems.

Asked what he thought leaders should do to tackle mental health issues, William told the Davos crowd: "I think more leaders need to talk about it."