Kate Middleton steps out in €1,200 floral Erdem dress at charity event with Princes William and Harry

William, Kate and Harry launched the next phase of their Heads Together mental health campaign by urging the nation to start a conversation with a friend, relative or stranger if they have a problem.

Speaking at a gathering of Heads Together supporters, partners and interested organisations in central London, the Duke said: "There are times when, whoever we are, it is hard to cope with challenges - and when that happens, being open and honest and asking for help is life-changing.

"Talking to someone else is a positive and confident step to take - but for too long it has been a case of 'keep quiet and carry on'. As a result, too many people have suffered in silence for too long, and the effects of this can be devastating.

"The three of us are really optimistic that things are changing. We believe that 2017 can mark a tipping point for mental health - a moment when more and more people no longer feel they have to bear the weight alone for fear of judgment."

The royal trio spearhead Heads Together, an umbrella group of eight leading mental health organisations, which is the charity of the year for the 2017 London Marathon.

William pledged to run the 26-mile race not in the UK but in Africa when he chatted to Good Morning Britain presenter Sean Fletcher, who will be taking on the challenge in London, but the broadcaster said Kate was a little sceptical.

Fletcher was among the invited guests who also included rapper Professor Green, real name Stephen Manderson, who has made programmes about the issue of mental health, Heads Together supporter and former England footballer Rio Ferdinand and representatives from companies like Facebook, Google and Royal Mail.

The broadcaster, who decided to run the gruelling race for Heads Together charity partner YoungMinds after his 13-year-old son was diagnosed with obsessive compulsive disorder, said of the Duke: "He said he will run a marathon sometime in Kenya.

"When I spoke to Kate she said she'd believe the Kenya marathon when she sees it."

William, Kate and Harry all spoke at the event held at the Institute of Contemporary Arts, which overlooks The Mall where the London Marathon will finish.

The Duchess, who wore an outfit by Erdem, outlined the next phase of the royal trio's mental health campaign to get people to open up about their issues.

She said: "What Heads Together is proposing is that in the weeks leading up to the Marathon, our campaign will showcase people from all walks of life, talking about the life-changing conversations that have helped them with their mental health challenges.

"We hope that these real-life examples will serve as encouragement to others to do the same.

"If we succeed with this, we will have taken a powerful step in normalising mental health as an issue in our society, thinking about it as we do our own physical health."

Harry praised the large number of people who be taking part in the marathon in support of their campaign on April 23.

He said: "Over 500 people will be running for Heads Together, leading from the front - well perhaps not right at the front - raising funds for the vital services provided by our charity partners. These runners will also be leading by example by starting conversations on mental health with their families, friends and colleagues.

"It has been unbelievably encouraging to see that attitudes towards mental health across the country are beginning to change. In the past, the phrase 'mental health' would be translated to mental illness. But thankfully that is changing."