NEW YORK (REUTERS).- A Francis Bacon self-portrait triptych is expected to fetch more than $25 million when it hits the auction block next month, Christie's said Monday.

"Three Studies for Self-Portrait," a 1974 work which depicts three distorted, guttural images of the British artist with eyes closed, against a dark background, are on display in London for the first time ever along with an untitled Bacon masterpiece known as "Crouching Nude on Rail" ahead of their sale in New York on May 11.

The nude, executed in 1952 by the then-emerging artist whose works in recent years have commanded some staggering prices, was among several works discovered in the 1990s in a London storeroom where Bacon had left them in the 1950s.

"The Bacon market is truly global and we have witnessed strong prices paid in recent months," said Brett Gorvy, Christie's deputy chairman and international head of post-war and contemporary art, who called Bacon one of the 20th century's greatest painters.

The cache also held one of Bacon's seminal Pope paintings, "Study after Velazquez."

Christie's estimates the abstract depiction of a half-man, half-animal figure will command in excess of $15 million.

A year after completing the self-portrait, Bacon said, "I loathe my own face, but I go on painting it only because I haven't got any other people to do ... (there is) nobody else left to paint."

Bacon's lover and frequent subject, the troubled George Dyer, died in 1971.

Bacon's triptychs are among his most celebrated and prized works. The record for the artist set in May 2008 for "Triptych, 1976," is $86.3 million. But with the market going into steep decline in late 2008 before bouncing back last year, works by the artist have seen uneven results.

"Study for Self-Portrait," a 1964 work estimated to sell for upwards of $40 million, failed to sell at Christie's in November 2008. But the $37 million fetched by Bacon's "Three Studies for a Portrait of Lucian Freud" at Sotheby's in London in February bolstered confidence. The price was more than double the pre-sale estimate.

Both works in the May sale come from unidentified U.S. collections.