It is a painting that has not been seen in public since 1963 and it may disappear again after it is sold next month for what is anticipated to be a world record price for the artist.
Study for a Portrait II, one of the famous series of popes by the British artist Francis Bacon, is expected to make in the region of £12m when it goes on sale at Christie's in London.
The work dates from 1956 and is one of the few of the acclaimed series inspired by Diego Velazquez's 1650 Portrait of Pope Innocent X not already in a museum or gallery.
Any collector keen to acquire a Bacon pope may decide to strike while the opportunity is there - despite the cost. Although the artist painted more than 50 papal portraits, he destroyed several of them.
Pilar Ordovas, head of post-war and contemporary art for the auction house, said: "It is very, very special. This is the most important work from Bacon's pope series to appear on the market and it will break records."
The study is presumed to be the second of two paintings of popes that Bacon completed in the autumn of 1956. The first is in the National Gallery of Canada.
It is unlike the dramatic and tempestuous "screaming popes" that Bacon painted earlier in the 1950s and presents, for perhaps the first time, a more sympathetic image. The period was an important turning point in his career as in 1956 he began visiting Morocco, primarily to visit Peter Lacy, a sadistic man with whom Bacon fell wildly in love. The work was last seen in public in a touring exhibition of British art in America in 1963. The present owner, who has not been named, has owned the work since the mid-60s.
Rising prices for the artist may have tempted the owner to sell. The current record for any work by Francis Bacon, $15m (£7.9m), was set at Sotheby's New York in November last year.
The study is one highlight of a series of important auctions in London next month. The largest evening sale of impressionist and modern art that Christie's has staged in London will be held on 6 February with a range of work estimated in the region of £75m.
The Bacon painting is among £40m of work on offer in the postwar and contemporary evening sale two days later.
The rival auction house Sotheby's, is putting a price tag in excess of £142m on its sales of impressionist and contemporary art for the week beginning 5 February, the highest estimate for any series of sales the house has staged in Europe.
Works from three major American collections are among the highlights.
The family of the late Charles R Lachman, a founding partner of the Revlon cosmetics giant, is selling three works including Deux Soeurs by Pierre-Auguste Renoir, which is expected to make £8m.
Paul and Mary Haas, Texan oil magnates and philanthropists, are selling works by impressionists including Cezanne. And a third collection, including works by Modigliani, comes from the estate of Herbert Singer, a New York lawyer, and his wife Nell.Reuse content