The overweight JobCentre manager whose nude portrait is set to fetch £18million

The 85-year-old artist's huge canvas is predicted to make up to $35million (£18million) for its owner, a European collector, at a Christie's sale on 13 May.

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Lucien Freud's painting of Sue Tilley is predicted to be sold for $35million

The auction house is putting the 1m x 2m picture on show at its King Street offices today, Monday and Tuesday, the first time it has been seen in public in Britain.

Today, the woman in the picture, 51-year-old Sue Tilley, who works in a West End JobCentre, told how it was almost never painted.

The first time the artist met her, in a Soho nightclub 20 years ago, he criticised her lipstick. "He said it had too many blue tones.

"The next time we met was over lunch at the River Café and I wore a different lipstick," she said.

She knew she was effectively being interviewed for the role of artist's muse and was briefed on how to behave by their mutual friend, performance artist Leigh Bowery.

"But I just did as I wanted as usual," said Ms Tilley, who grew up in Sussex Gardens and now lives in Mornington Crescent.

"Soon after, 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.

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"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.

"Leigh got rubbed out because he went to Scotland and Lucian needs you to be devoted. He won't work on the painting when you're not there."

Ms Tilley, who has worked at the JobCentre in Denmark Street for 30 years and previously worked in nightclubs, modelled for Freud on her days off.

"For the picture on the sofa I only had three days off in two years. Every weekend he expected me to be there and I couldn't go on holiday because I couldn't get a tan," 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 she was given by Freud, whom she has lost touch with, for £26,000.

But she was astonished by the estimate Benefits Supervisor Sleeping, 1995, has attracted.

"I'm beside myself. Wait till I tell my mum," she said. "If I had the money I'd buy it myself because it's my favourite."

She added: "I might go to the exhibition. I'm sure no one would recognise me."