Kate sparkles on first night out without baby Prince

The Duke and Duchess at the inaugural Tusk awards that celebrate the work of wildlife conservationists. Photo / AP

The Duchess of Cambridge sparkled on her first night out since she became a mother seven weeks ago, choosing a sequinned dress to show how quickly she has regained her figure.

The Duchess wore a glittering sleeveless creation by Jenny Packham, one of her favourite designers, as she accompanied her husband, the Duke of Cambridge, to a conservation awards dinner in London yesterday.

She completed her outfit with Jimmy Choo "Vamp" sandals and a diamond bracelet understood to have been a wedding gift from the Prince of Wales.

The couple told guests it was the first time they had left baby Prince George at home with his nanny Jessie Webb, 71, who looked after Princes William and Harry when they were boys.

The couple were attending the inaugural Tusk Conservation Awards, celebrating the work of wildlife conservationists, on the day that the Duke announced he was leaving the armed forces to concentrate, in part, on protecting endangered species.

In a speech during the event at the Royal Society, the Duke said: "As you might have gathered, Catherine and I have recently become proud parents - of a baby who has a voice to match any lion's roar.

"This is actually our first evening out without him, so please excuse us if you see us nervously casting cheeky glances at our mobile phones to check all is well back home."

Turning to the theme of the awards, he went on: "Like any new parents, our thoughts inevitably turn to the world our child will inherit. It is unfathomable to imagine a world in which children who have been born in the past couple of months may grow up in a world in which rhinoceros have ceased to live in the wild." He said that in the past year 35,000 elephants had been killed for their ivory, a statistic he called "truly scary".

He went on: "The possibility of extinction is bad enough for one of our children growing up here in the West, who will never experience the magic of seeing a rhino; or even for my own little George, who Catherine and I very much hope to introduce to east Africa - a place we know and love - in the fullness of time.

"But for a child growing up in Africa and whose birthright and economic inheritance these creatures are, it is [immoral] that he or she may never experience what his parents and grandparents ... treasured."

It was only the second public engagement for the Duchess since she gave birth in July.

Last month she made a surprise appearance by her husband's side as they started a coastal marathon on Anglesey.

Charles Mayhew, founder of Tusk, of which the Duke is patron, said the Duke was "really excited" about the awards ceremony.

"You have seen tonight the passion he has for conservation. He has had that since he became patron of Tusk in 2005.

"Since the birth of Prince George, like all new parents, you tend to take stock and look at the world in a different light and think about what are we leaving our children, what is the legacy."

He added: "We arranged this date even before the Duchess became pregnant, so it is completely fortuitous that we have the Duchess here for her first night out since the baby was born and on the day that the Duke has made such a big announcement about his future."