A nude by Lucian Freud that took 16 months to paint sold for 13.9 million pounds ($18.2 million) at a London auction on Wednesday, the first time the work, hailed as a "masterpiece," has come to market.

Works by the British artist, who died in 2011, have attracted growing interest – and prices – in recent years.

"Ria, Naked Portrait" was the star attraction of Wednesday's sale, fetching 11,810,00 pounds, rising to 13,891,500 pounds once the buyer's fee is added on.

"It's a late masterpiece," Anna Touzin, from auctioneers Christie's, told Agence France-Presse (AFP). "It was painted between 2006 and 2007 and in the same collection since it was made.

"It's the first time it's coming to the market, which is really exciting."

Freud, considered one of Britain's greatest portrait artists, took 16 months to paint the work, with the subject – Ria Kirby – sitting virtually every day for up to five hours.

"So a really long process and I think it shows Freud's relationship with his sitters, the kind of dedication that they would give to him but that he also gave to them, painting them," Touzin added.

Freud – the grandson of psychoanalyst Sigmund Freud – is known for his uncompromising nudes and self-portraits, replete with flabby breasts or rounded stomachs.

"He doesn't want to portray an idealized view of the body ... he's really trying to paint people as they are," said Touzin.

That has made Freud one of the most sought-after artists in the art market.

In 2022, his "Large Interior, W11" sold for a record $86 million; in 2015, another nude, "Benefits Supervisor Resting," went for more than $56 million.

The Christie's sale also included Freud's "Head of a Woman," which sold for 4,187,880 pounds.

Works by Marc Chagall, Berthe Morisot, Willem de Kooning and Jeff Koons also went under the hammer.