A security guard stands in front of Triptych 1974-1977 at Christie's

Triptych 1974-77 is valued at �25 million but is expected to exceed estimates when it appears at auction for the first time.

It is the last in the series that Bacon painted in response to the suicide of his lover George Dyer in 1971.

The auction will take place at Christie's on 6 February in London.

Dyer had committed suicide on the eve of the opening of Bacon's last retrospective in Paris, in the hotel room he shared with the artist.

Many of Bacon's works after this moment were preoccupied with his lover and the tragic manner of his death.

Triptych 1974-77 shows sequential images of dark, ominous umbrellas and Dyer struggling on a near-deserted beach.