Nicole's party frock shock

From the front, there was nothing remarkable about her appearance.

But when Nicole Kidman turned round, it became startlingly apparent that this was no ordinary little black dress she was wearing.

With a gaping expanse of back on show, the actress stunned onlookers as she stepped out in the daring creation for a high-profile New York party.

The plunging velvet dress was the talk of the evening when she greeted photographers at Vanity Fair magazine's annual Tribeca film festival.

The 37-year-old star, who is officially single but rumoured to be dating billionaire Steve Bing, wowed onlookers with her sleek - and revealing - appearance and her platinum blonde hair tied back. A fellow partygoer said: 'When Nicole arrived, everyone thought she'd simply opted for a fairly conservative little black dress.

'She already looked great from the front, but when she turned around to pose for photographers, jaws dropped as people realised just how low-cut her dress really was.

'She was the talk of the party. But in keeping with her lean figure, Nicole's back and arms looked amazingly toned.'

Miss Kidman is currently promoting her latest film, The Interpreter, a thriller in which she stars with Sean Penn.

She is quick to emphasise her single status, saying: 'I've been single for ever. If I'd truly been with all those men, I'd be exhausted. My 90-year-old grandmother rings me from New Zealand and says, "I read in a magazine that you're getting married," and I'm like, "Grandmother would you stop it? It's not true."'

The Australian actress, who earns more than £2million a year as the face of Chanel, works hard to maintain her slim figure and fresh complexion. She has adopted a stringent beauty regime to ensure that she sparkles whenever she is seen in public.

Before the UK premiere of The Interpreter in London last week, she spent the day pampering herself in preparation.

After a morning workout in her suite at the Dorchester, Miss Kidman - a keen jogger and yoga devotee - also spent three hours in the hotel's spa.