William and Kate at mental health summit

They smiled for the crowds but the Duke and Duchess of Cornwall understood the depths of despair that underpinned the occasion.

The Duke and Duchess of Cambridge during the Global Ministerial Mental Health Summit at County Hall in London.

The two-day Global Ministerial Mental Health Summit had been convened to discuss how to deal with the stigma attached to mental health – a subject that is also the subject of William and Kate’s Heads Together campaign.

The event in London coincided with a finding by the Mental Health Foundation that one in 10 children felt they had no one to talk to if they felt worried or sad.

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Appropriately, the Royal couple viewed, and were invited to apply the finishing brushstroke to, a picture by the Colombian artist Dairo Vargas, called Someone To Turn To.

William and Kate were greeted by the Health Secretary, Matt Hancock, as they arrived to cheers from crowds waiting outside County Hall. Inside, they heard discussion of a declaration committing to putting mental health on an equal footing with physical health.

The summit has been arranged for today’s World Mental Health Day, which is also the focus of the Make It Count campaign for every child in the UK to receive an education with mental health at its heart.

Dr Antonis Kousoulis, associate director at the Mental Health Foundation, said there was a “growing crisis” in the mental health of children.

She said: “Nearly half a million children in the country have no one to speak to at school when they are experiencing feelings of sadness or worry. That is plainly unacceptable.”

Alexandra Almeida, from the My Mind Our Humanity campaign – part of the Lancet commission on global mental health – said it was “very nice” to meet William and Kate and have their support.