The normally-sedate Kate drew gasps of delight from the crowd as she arrived for Gary Barlow's Prince's Trust concert in a black and white mini dress.
Gary met Kate, Prince William, the Prince of Wales and Camilla before the Albert Hall gig.
Then he belted out a selection of Take That's greatest hits ? as the Royals clapped along with the delighted audience.
Lulu, Jason Donovan and Olly Murs all joined The X Factor judge onstage.
Gary, 40, said he did not suffer any pre-show jitters.Rock and Royal ... Kate and Wills meet Gary backstageArthur Edwards/ The Sun Star ... Gary Barlow belts out classicsArthur Edwards/ The Sun Anything you can do ... the Royal party sing the national anthem before Gary's concertArthur Edwards/ The Sun
He added: "I wasn't nervous. I really loved it."
The Trust is raising cash so it can help areas of London, Manchester and Birmingham hit by the riots.
By ERICA DAVIES, Executive Fashion EditorDazzling ... the Duchess of Cambridge wore a short Zara dress and simple black jacketSplash News
SHE is fast becoming the queen of appropriate dressing. And last night Kate did not put a formal foot wrong.
She opted for a very short Zara printed dress to show off those famous legs, teaming it with a simple black Ralph Lauren jacket.
Kate even ditched her favourite nude shoes for a very high pair of black heels.
It isn't what most people wear to a gig. But for a night with the in-laws she looked chic, polished and a bit more rock and roll than usual.