Head on a Green Sofa, an iconic work by Lucian Freud, is to auction at Sotheby's with an estimate of £2.5m-3.5m ($4m-5.7m).
The work will headline the London Evening Sale of Contemporary Art on February 12.
Oliver Barker, Sotheby's senior international specialist for contemporary art, commented: "This exceptional painting has everything. It is one of only a very small number of great Freud paintings that have never before surfaced on the market, and it beautifully captures the form of the woman who was to play such a central role in Freud's life for over two decades.
"It also marks a moment of cataclysmic change in Freud's artistic style and practice and sees him embark on a new way of painting which was to define his career."
The 1960-61 portrait depicts Lady Belinda Lambton, with whom Freud enjoyed a lifelong friendship.
The wife of UK Conservative minister Lord Lambton, she and Freud were kindred spirits - with Lambton introducing the artist to members of her illustrious social circle and constantly championing his work.
Lord Durham, the son of Lady Lambton, who is consigning the painting in the wake of an inheritance settlement brought by his sisters, shared his memories of the famously cantankerous Freud: "My earliest memory of Lucian was when my mother took me to his studio.
"I couldn't have been much more than about five years old. He was painting. I shone a torch in his face. He was furious and screamed at me. I never forgave him."
A painting by Francis Bacon, another friend of Freud's, will auction with a £30m ($49.2m) estimate at Christie's London on February 13.
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