Kate Middleton: A knockout in the ring

The Duchess of Cambridge tries out boxing at the launch of the Heads Together campaign on mental health in London. Photo / Getty

The Duchess of Cambridge proved she could pack a punch on a royal engagement yesterday - as she picked up a pair of boxing gloves and started sparring.

Kate, 34, faced three-times world champion Duke McKenzie. Prince William and Prince Harry couldn't resist following suit. And while Muhammad Ali stung like a bee, Harry apparently "purred like a Bentley".

McKenzie said afterwards: "Harry had the strongest punch. He's a banger make no mistake. He runs like a Bentley - he purrs quite well."

Harry was dubbed a "banger" by boxing legend Duke McKenzie. Photo / Getty

Talking about Kate, boxer McKenzie said: "She was lovely, very gentle. She's better than the pair of them as she's more technical. She had a mean right jab and a wicked punch."

The trio were in Stratford, East London, to launch a mental health campaign called Heads Together. Devised by the three royals, it brings together leading mental health charities to encourage people to speak out and seek help - for example by trying a new activity like boxing to relieve stress.

Kate's boxing ability outshone William and Harry's. Photo / Getty

Jade Jennings, 47, who attends McKenzie's boxing classes, revealed: "Harry told me when he's angry he likes to have a good box."

And ten-year-old Maggie, who spoke to Kate, said: "I'm shy and the princess said she was too."

The royals were also asked to say what "made them happy".

Prince William couldn't match his wife's technique. Photo / Getty

Harry characteristically said "making people smile", William wrote "my children" - and Kate said "going for a walk".