LONDON (Reuters) - Prince William, his wife Kate and brother Harry urged Britons on Monday to ditch their prejudices about mental health in what is being billed as the biggest joint project the three young royals have taken on.

In the "Heads Together" campaign they launched in London, the Duke and Duchess of Cambridge and Prince Harry will work with charities "to change the national conversation on mental wellbeing", a note from the couple's Kensington Palace residence said.

"We have seen time and time again that unresolved mental health problems lie at the heart of some of our greatest social challenges," William said in a joint speech with Kate and Harry.

"... Heads Together wants to get people talking. The more we talk about mental health, the more normal the topic becomes, and the more we feel able to open up and seek support."

Harry played block-stacking game jenga, which is used by some mental healthcare professionals as a therapeutic tool, while Kate and William tried some boxing moves to illustrate the role exercise is thought to play in promoting mental wellbeing.

Kate has been vocal about mental health issues in the past. In February, she used a guest editor slot at the Huffington Post UK news website to commission articles on mental health issues affecting children.

(Writing By Marie-Louise Gumuchian; editing by John Stonestreet)