(Reuters) - A 1962 painting by artist Francis Bacon of his lover Peter Lacy is expected to sell for as much as $40 million when it is auctioned in New York next month, Sotheby's said on Monday.

The oil on canvas "Study for a Portrait of P.L." is considered one of Bacon's most significant works and will be among the highlights of Sotheby's contemporary art evening sale on May 14, the auction said.

"He was the great love of Bacon's life," said Oliver Barker, Sotheby's deputy chairman, Europe. Lacy moved to Morocco in the mid-1950s, and Barker said Bacon received the news of his death among congratulatory telegrams for the opening of his famous 1962 Tate exhibition in London.

The painting in blue, green and black depicts Lacy seated on a bench holding a glass of wine. It was executed posthumously months after Bacon learned of his lover's death from alcohol abuse. Bacon, who died in 1992, painted many of his subjects from photographs and memory.

Barker said the work shows a new direction for Bacon's work and the influence of other artists.

"It marks a kind of sea change in terms of the visual language of Bacon's painting after 1962. This is a very radicalized setting, or composition, for the painting with its horizontal treatment of the bench underneath the figure, but also his very American Abstract Impressionist-inspired color field and painting background."

Bacon met Lacy, a former Battle of Britain pilot, in 1952 in London. Their tempestuous relationship endured throughout the 1950s.

"If you look at all of Francis Bacon's works, and there are not more than around 600 paintings in existence, many of them are devoted to people who were in a very small coterie of friends and colleagues," Barker said. "Peter Lacy is one of the most pivotal."

The painting, which is being sold from a private collection, has not been seen in public for about 40 years. It will be displayed in London from April 12-16 and in New York from May 3.

Barker said the pre-sale estimate for the work reflects interest in Bacon's work since the correction in the art market in 2008. The record for a Bacon work sold at auction is $86.3 million, which was set at Sotheby's in New York in 2008 for "Triptych, 1976," a work based on Ancient Greek mythology and the legend of Prometheus.

"The world's greatest art collectors have woken up to the fact that Bacon is arguably one of the most important painters of the last 100 years," Barker added.