Kate Moss by Lucian Freud Enlarge Image

Entitled Naked Portrait 2002, the almost life-size painting captures the model when pregnant with her daughter Lila Grace.

The painting is part of a post-war and contemporary art sale being held at Christie's auction house in London.

The painting was sold to an anonymous bidder by telephone. It had been expected to fetch �3.5m.

Famous model

Moss first suggested she pose for Freud in an article in Dazed and Confused magazine.

Lucian Freud, grandson of famed psychiatrist Sigmund Freud, usually paints people he knows well but his most famous model, aside from Moss, is probably the Queen.

It takes between six months and a year of regular sittings for the artist to complete a painting, but Moss said she was unconcerned by the length of time it would take.

Her daughter Lila Grace, from a previous relationship with publisher Jefferson Hack, is now two years old.

The model was thought to be among the painting's potential buyers.