LONDON.- A painting by Irish-born artist Francis Bacon sold for 18.0 million pounds ($28.7 million) on Tuesday, the second highest price paid for a work of art at a Christie's post-war and contemporary auction in London.

"Study for a Portrait," depicting a besuited man seated on a gilded armchair enshrouded in a sea of blue, had been expected to fetch around 11 million pounds, although the sale price includes a buyer's premium which the estimate does not.

The most expensive work of art sold at an equivalent sale at Christie's, London, was also by Bacon -- his "Triptych" raised 26.3 million pounds in 2008.

Executed in 1953, between Bacon's famous Pope series that year and his Man in Blue paintings of 1954, "Study for a Portrait" has never come to auction before.

Rodrigo Moynihan, who lent Bacon a studio, was the first owner. It later belonged to Louis Le Brocquy, the renowned Irish painter, who was the last to keep it before its acquisition by the present owner in 1984.

Elsewhere at the auction, "Woman Smiling" (1958-59), a landmark portrait by Lucian Freud, sold for 4.7 million pounds.

The only single portrait of Suzy Boyt, the woman who was to mother four of the artist's children, was last sold at auction in 1973 when it realized 5,040 pounds.

A large scale portrait of Chairman Mao by Andy Warhol dated 1973 fetched 7.0 million, in line with expectations.

Overall the auction raised 78.8 million pounds. Rival auction house Sotheby's holds its equivalent sale on Wednesday during a key few weeks for the art market, which has rebounded strongly from the slump of 2009.

(Reporting by Mike Collett-White)