Sue Tilley posed for Freud over a four-year period in the early 1990s

A life-sized Lucian Freud painting of a sleeping, naked woman has set a new world record price for a work by a living artist.

The 1995 portrait, titled Benefits Supervisor Sleeping, sold for $33.6m (£17.2m) at Christie's in New York.

The previous record was held by Jeff Koons' Hanging Heart, which fetched $23.5m (£12.1m) last November.

The Freud work, sold at auction for the first time, shows Jobcentre supervisor Sue Tilley, now 51, asleep on a sofa.

Christie's described it as a "bold and imposing example of the stark power of Freud's realism".

Ms Tilley, nicknamed "Big Sue", was introduced to the painter, now 85, by the Australian performance artist Leigh Bowery.

She posed for Freud for four years in the early 1990s, and told BBC Radio 5 Live it was a strange feeling posing in the nude.

She said: "The first couple of times, I was a bit embarrassed but you get used to it. It's a bit weird to think that a picture of me could be worth so much money.

"You don't have to sit still the whole time. It's two or three days a week and you have breaks."

She added: "When we were painting it we didn't sit there going: 'I bet this'll be the biggest selling painting in the world'. It was just like one of his other pictures."

The previous auction record for a Freud painting was $19.3m (£9.9m), paid in November for his 1992 work IB and Her Husband.