An important painting by the British painter Frank Auerbach of his long-term lover, Estella Olive West, entitled Head of E.O.W. III, 1961, is to be sold at Bonhams as part of its 20th Century British Art sale.
The painting, which has never appeared at auction before, is expected to bring £400,000-600,000 at the London auction on June 30.
Estella, a widowed amateur actress, was Auerbach's lover for 23 years and one of his most regular models, referred to as EOW in the titles of his works.
It was with Estella that he started the process of working and reworking paintings, which sometimes took hundreds of sittings to complete.
Estella once said of being a sitter: "It was quite an ordeal because he would spend hours on something and the next time he would scrap the whole lot down.
"That used to upset me terribly. I wondered what I was doing it all for."
Auerbach was born in Berlin in 1931 and emigrated to England in 1939.
He is best-known for his paintings depicting female models; including Estella, his wife Julia and a professional model Juliet Yardley Mills, as well as his scenes of London, especially Camden Town.
A rare charcoal and chalk on paper by Auerbach will also feature in the sale. Entitled Portrait of Christopher Dark, 1977, it has attracted a pre-sale estimate of £40,000-60,000.