Freud's 'Big Sue' expected to fetch £18m at auction

A life-size painting by Lucian Freud which has never been seen publicly in Britain is expected to sell for up to £18m, making it the most expensive work by a living artist at auction.

The work, Benefits Supervisor Sleeping – which features a Jobcentre worker fondly known as "Big Sue", the subject of several masterpieces by Freud – was painted in 1995. Because it was sold privately at the time, it has never before appeared at auction.

Christie's, which is selling the piece in New York next month, called it "the most important work by Lucian Freud" and predicted it could smash the current world auction record for a work by a living artist, held by Jeff Koons's Hanging Heart which fetched £11.3m last year. Before then, Damien Hirst had held the title for Lullaby Spring, which sold for £9.6m, also last year.

The portrait's subject, Sue Tilley, is now 51. She posed for Freud for four years in the early 1990s after being introduced to him by the Australian performance artist Leigh Bowery, who was himself the subject of several large scale portraits by Freud in the 1990s. Bowery recommended several sitters, attempting to find models whose dramatic physicality would appeal to him. Freud, 85, had responded with great enthusiasm to Bowery's "bulky" form and that is what is said to have led the performance artist to consider his friend as a potential model.

Freud first painted the subject in 1993, in a work which shows her lying in an uncomfortable pose on the bare floor.

Ms Tilley, who lives in London and posed for Freud on her days off from her job at the central London Jobcentre, said it was her "favourite" work and would buy it herself if she could. She has previously speculated about why she may have been selected by Freud as a model, saying: "I think he probably picked me because he got value for money. He got a lot of flesh."

Speaking of when she wentfor her first sitting, she said: "I was mortified when I got there and he told me to take my clothes off."

Before the artist started working on the portraits of Ms Tilley, he covered her tattoos with fresh-coloured paint, because "he adores flesh so much."

The auction record for a Freud is £9.3m, set in November 2007 for IB and her Husband.