Oliver Barker, senior international specialist for contemporary art at Sotheby’s, hailed the painting as “exceptional”, representing “exactly the moment” when Freud adapted a new technique working standing up using coarse hogs’ hair brushes.

He added the one-metre-square painting showed Freud “luxuriating” in his new liberations, focusing on the “joy and freedom” of painting.

“This exceptional painting has everything,” he said. 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."