The Triptych is an important work for several reasons. It has rarely been seen in public; it is the only triptych of Dyer ever to be auctioned; and it is the first time Bacon used photographs by his friend John Deakin – a former Vogue staff photographer - as source material for an artwork.

Bacon hardly ever painted from live models. Bacon rarely painted from live models. Talking of his subjects, Bacon said: “If I know them and have photographs of them, I find it easier to work than actually having their presence in the room. I think that, if I have the presence of the images there, I am not able to drift so freely as I am able to through the photographic image. This may just be my own neurotic sense but I find it less inhibiting to work from them through memory and their photographs than actually having them seated there before me.”

Bacon works are now some of the most expensive that exist. His large-scale triptych Three Studies of Lucian Freud (1969) broke all auction records when it sold for $142m in New York last November. In February a portrait of Dyer became  Christie’s highest fetching lot in Europe as Portrait of George Dyer Talking (1966) sold in London for £42m, setting an auction record for a portrait of Dyer, and the most valuable work of art that Christie's had sold in Europe. The seller Mexican financier David Martinez Guzman was said to have purchased the painting in a private sale for $12m. A small-scale triptych – of Freud again – was sold for £23m in 2011. Oliver Barker, Sotheby’s Senior International Specialist in Contemporary Art, said, about the work on sale: “Painted less than a year after their first encounter, Three Studies for Portrait of George Dyer marks both the height of Bacon’s affair with Dyer and the zenith of his achievement in portraiture. Full of the painterliness, chaotic brushstrokes and raw emotion that make Bacon such a giant among artists, we expect it to create great excitement at auction, coming at a moment when the market for works by Bacon is at an all-time high.”

Not seen in London for over 20 years, Three Studies for Portrait of George Dyer (on Light Ground) will be exhibited in Sotheby’s London galleries from Friday 27th until noon on Monday 30th June, prior to its sale on the evening of the 30th.

See more work by Francis Bacon on Barnebys here.