A life-size Lucian Freud painting of a naked benefit office supervisor sleeping is expected to smash auction records next month.

The artwork, which is being sold by a private European collector, is predicted to fetch up to $35 million (£18 million) when it goes on sale in New York.

Christie’s believes that Benefits Supervisor Sleeping will beat the world auction record for a work by a living artist, held by the American artist Jeff Koons’s Hanging Heart (Magenta/Gold), which fetched $23.5 million when it was sold last year.

The Freud painting, which dates from 1995 and is appearing at auction for the first time, depicts a rotund civil servant from London, Sue Tilley, now 51, sleeping on a dilapidated sofa. She had been introduced to Freud by the Australian performance artist Leigh Bowery.

At 20 stone she seemed an unlikely choice of muse for an artist, but Freud has spoken of his “predilection towards people of unusual or strange proportions”. Referring to the woman he affectionately calls “Big Sue”, he said that he was “very aware of all kinds of spectacular things to do with her size, like amazing craters and things one’s never seen before”.

Freud, 85, first used Ms Tilley as a model for a painting titled Evening in the Studio (1993), for which she had to lie in an uncomfortable pose on a bare floor. He then bought the ragged sofa on which she is shown lying in the 1995 painting.

“I think he probably picked me because he got value for money,” Ms Tilley once joked. “He got a lot of flesh.” She believes that he was inspired by her “ordinariness”.

For most of the works depicting her, Freud covered Ms Tilley’s tattoos with flesh-coloured paint, because, as she has said: “He adores flesh so much, and to have sort of green flesh and pink flesh isn’t really normal.”

Freud gave her one of the portraits, a print. When bailiffs visited Ms Tilley some years ago, demanding items to the value of £700, they were more interested in her electric kettle and household objects than her Freud. When, in desperation, she offered to part with the print, telling them that it would more than cover the money she owed, they laughed at her. In 2005 it was sold by Bloomsbury Auctions in London for more than £26,000.

The artist, whose sitters have included the model Kate Moss, said: “I paint people not because of what they are like, not exactly in spite of what they are like, but how they happen to be.”

Pilar Ordovás, head of postwar and contemporary art at Christie’s London, said: “Benefits Supervisor Sleeping is one of the finest of Freud’s paintings from the 1990s. The work is a bold and imposing example of the stark power of Lucian Freud’s realism.

“This picture is a simple and seemingly uncomposed depiction of one of the key features of Freud’s art: the forceful and undeniable physical presence of people and things.”

The painting, which was in a private European collection, is making its auction debut. It will be displayed in Britain for the first time today at Christie’s in London before its sale in New York.

The auction record for a painting by Lucian Freud is the $19.3 million achieved in November 2007 for Ib and her Husband, from 1992, which features Freud’s daughter Isobel. Benefits Supervisor Sleeping is expected to fetch at least $25 million at Christie’s New York postwar and contemporary art evening sale on May 13.

Fat as an artistic issue

— Early Man saw obesity as a symbol of fertility. Venus of Willendorf is a 25,000- year-old statuette found in Austria in 1908, whose heavily pronounced breasts and swollen belly symbolise her roles as a child-bearer

— Peter Paul Rubens (1577-1640) portrayed obesity as a symbol of beauty and pleasure.

— A number of his paintings, including

— The Bacchanal and Venus at the Mirror, above, convey fatter women as voluptuous and sexually attractive

— William Hogarth (1697-1764) often used obesity as a symbol of the high social status of his subjects. The Bench portrays several overweight judges presiding over a case. Their weight is used to illustrate their laziness and gluttony, which works only because of their high social class

— Beryl Cook (b 1926) consistently uses obesity as a subject of her largely comic paintings. In works including Tea in the Garden (2003) groups of figures enjoy normal activities while subverted by their nakedness and obesity

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