Kate Middleton, Prince William and Prince Harry set to be part of mental health documentary

The Duchess of Cambridge, née Kate Middleton, Prince William and Prince Harry will be working with presenter Nick Knowles, who has collaborated with the two princes in September 2015 when they helped build homes for injured or sick servicemen in Manchester.

“I’m working on a mental health documentary with the young royals, which should come out around the time of the Marathon,” he told The Sun. “I can’t tell you anymore. I’m not even sure I should have told you that.”

The BBC documentary will reportedly air in the lead-up to the London Marathon in April.

The Heads Together campaign was launched by the three royals in May 2016 in an effort to fight the stigma against mental health issues.

Beneficiaries and members of its mental health charities will be running at the London Marathon in April.

The Kensington Palace and the BBC have yet to comment on the documentary.