A "seductive and sexually charged" Francis Bacon portrait is expected to fetch around £18 million when it goes on sale next month.
The Portrait Of Henrietta Moraes is the highlight of an auction of post-war and contemporary art at Christie's in central London on February 14.
The painting, which dates from 1963, has been owned by a private collector for almost 30 years.
Francis Outred, Christie's head of post-war and contemporary art, Europe, said: "Searing with raw colour and texture, Portrait Of Henrietta Moraes is one of the most seductive and sexually-charged paintings I have ever encountered by Francis Bacon.
"The carefully constructed mood through colour is forcefully invaded by the extraordinary swipes of the loaded brush, which create the woman's voluptuous figure. This juxtaposition of the sheer beauty of colour with the brutal physicality of paint is what makes Bacon's art so remarkable."
The portrait was painted a year after the artist's breakthrough exhibition at the Tate Gallery in London.
Mr Outred said: "The work has only had two owners since the day it was made, one of which was the important collector and post-war industrialist Willy Schniewind and the other being the present owner, a distinguished New Yorker who acquired the work in 1983.
"Portrait Of Henrietta Moraes has not been seen in the public eye for 15 years and I am very excited to be presenting this important piece of British art in London."
The model in the painting, Henrietta Moraes, was a friend of Bacon's and a well-known party girl on the Soho painting scene in the 50s and 60s. She was also a muse to Lucian Freud, who painted her in the early 50s. She died in London in 1999.