A "sensuous" portrait by Picasso of his mistress at the height of their passion has been sold at auction for more than �15m.

Nu au collier, fetched �15,956,650 - almost double the estimate of up to �9m - when it went under the hammer at Christie's in London.

The sale certainly outstripped all pre-sale expectations and has become the most expensive painting sold in London this year

Christie's spokesman

The painting depicts Marie-Therese Walters, who Picasso met in the street when she was 17, and was last shown in public in the 1950s.

The work was the main attraction at Christie's annual sale of impressionist art, which also includes two more multimillion pound Picassos.

The painting was purchased by a bidder in the room

Unknown vendor

"The sale certainly outstripped all pre-sale expectations and has become the most expensive painting sold in London this year," a spokesman for Christie's said afterwards.

The painting was sold by a private European collector who does not want to be identified.

Before the auction it had been hoped that the bidding might reach between �6m to �9m if two people in the room were both trying to outbid each other.

Nu au collier was painted in 1932, five years after the artist first met Walters on the street and reportedly said: "I am Picasso. You and I are going to do great things together."

Christie's spokeswoman Alexandra Kindermann said: "Nu au collier dates from the height of their love affair and the fluid, sensuous forms of the painting reflect Picasso's love and desire for his young and beautiful muse."

World record

It is an unusually vibrant artwork, according to Christie's director of modern art, Conor Jordan.

"This painting is lyrical. That is the only word to describe it," he said.

"The eyes are open - which is unusual for pictures of Marie-Therese - so there is much more engagement with the viewer.

"The lines are voluptuous and the use of high colour greens and lilacs are trademark for his paintings of her."

Despite fears of a downturn affecting the art market generally, "blockbuster" lots in the US spring auctions of modernist and impressionist art have done well.

The world record for a Picasso is $55m (�37.9m) for Femme aux bras croises, from his Blue Period, sold in November 2000.