A painting of a naked JobCentre manager by Lucian Freud is expected to become the most expensive picture by a living artist next month.
The life-size canvas, entitled Benefits Supervisor Sleeping, is predicted to make up to £18 million at Christie's in New York on May 13.
Sue Tilley, the subject of the 1995 painting, was introduced to Freud by their mutual friend, Leigh Bowery, a performance artist.
The artist, now 85, referred to her affectionately as "Big Sue" and she became one of his favourite models. She has said of being Freud's muse: "I think he probably picked me because he got value for money. He got a lot of flesh."
Miss Tilley, 51, who works at a JobCentre in Soho, central London, told yesterday how she suffered for Freud's art.
"Leigh called me up and said, 'Lucian wants you to start work next week' and he made me practise stripping off on my settee at home before I went to Lucian's house in Holland Park. I like a bit of excitement but I was still mortified when I got there and he told me to take my clothes off.
"For the first picture I had to lie on the floorboards in a most uncomfortable position with Leigh and Nicola, the woman he married, and a dog."I was in agony and I thought about giving up. But we work hard in my family so I stuck it for the whole nine months," Miss Tilley explained.
"Every weekend he expected me to be there and I couldn't go on holiday because I couldn't get a tan."
Despite the inconvenience, Miss Tilley looks back on the experience with fondness.
She said: "It was wonderful to spend time with such a fantastic person and see him working. Lucian has an opinion on everything and he was interested in my run-of-the-mill life."
Three years ago, she sold an etching of herself by Freud for £26,000. But she said she was astonished by the estimate for the painting. "I'm beside myself," she said. "Wait until I tell my mum. If I had the money I'd buy it myself because it's my favourite."
The current world auction record for a work by a living artist is held by Jeff Koons's Hanging Heart (Magenta/Gold), which sold for £11 million last year.