Portrait of Bacon's grief expected to sell for £26m

The most important triptych by Francis Bacon ever to appear on the open market, which reveals the artist's grief over his lover's suicide, is to be auctioned next month at an estimated price of £26m.

Such a price for Triptych 1974-77 would match the current record for the most expensive Bacon work.

The piece, which is the fourth and last in the Black Triptychs series, focusing on Bacon's grief after George Dyer killed himself in Paris in 1971, differs from the others in expressing a sense of hope. None of the other triptychs by the artist, who died in 1992, has ever appeared on the open market and only one is in private hands. The sale will take place in London on 6 February.

The stormy relationship between Bacon and his lover is said to have begun after the artist caught the rugged East Ender burgling his Kensington apartment in 1964.

It ended, less than a decade later, in the Paris hotel room where Dyer took an overdose on the eve of a major retrospective of Bacon's work at the Grand Palais. Both men had been due to attend the exhibition's opening the following day but Bacon returned to the hotel room, after leaving his lover on his own for a few hours, to find Dyer dead.

Bacon became obsessed by his lover's memory and over the next few years produced works that documented the anguish he felt over his loss.

Triptych 1974-77 is regarded as a seminal artwork showing his lover writhing on a near deserted beach overlooked by the spectre of two terrifying monochrome Orwellian witnesses, and became the centrepiece of an exhibition at the Metropolitan Museum of Art in New York. Bacon painted it for the show as a tribute to Dyer. Pilar Ordovas, head of post-war and contemporary art at Christie's, said it was one of the finest and most mysterious Bacon paintings from the 1970s, produced when the artist was at the height of his powers. "The other three have very sombre black backgrounds and while this is very tortured and incredibly strong, there is also some sort of hope, with the sea and the horizon in the background," she added.

In interviews with his friend and critic David Sylvester, Bacon said the painting of Dyer in the four years after his death was a "conscious act of exorcism".

In Michale Peppiatt's book on Bacon, entitled Anatomy of an Enigma, the artist is quoted as saying: "I feel profoundly guilty about [Dyer's] death. If I had never gone out that morning, if I had simply stayed and made sure he was alright, he may have been around now. He had tried to do it before. He changed completely with drink."

Bacon also said he wished he had left Dyer "exactly the same as when I first met him".

"By giving him enough money to do nothing, I took his incentive away. Stealing gave him something to think about. . . I thought I was helping him."

The current record for any work by Bacon is the £26.6m paid for Study from Innocent X at Sotheby's in New York in May last year.