A Francis Bacon portrait is expected to fetch £7.5m when it is sold at Christie's auction house in London.
The 1969 work is of socialite Henrietta Moraes, who sat for several artists, was one of a small number of women that Bacon painted.
Christie's Pilar Ordovas said the "wonderful" portrait "offers a fascinating insight into life and the characters of 1960s Soho".
The post-war and contemporary art sale takes place on 19 October.
Ordovas became a close friend and Soho drinking partner of Bacon during the 1950s, before he found fame.
The model, who died in 1998, inscribed the reverse of the canvas which is being sold.
She was also painted by Lucian Freud.
The work has been owned by Garech Browne, a member of the Guinness brewing dynasty, since 1970.
Mr Browne went on to become a patron of the arts.
In May Bacon's Triptych masterpiece broke the artist's record at auction after selling for $86.3m (£43m) in New York.
Bacon, who died in 1992, is currently the subject of a retrospective at Tate Britain.