LONDON (REUTERS).- Christie's auctioneers will offer a portrait by Lucian Freud which it described as "seminal" in the career of the British artist at a sale in London on June 28.

"Woman Smiling," painted in 1958/9, is the only single portrait Freud made of Suzy Boyt, the woman who mothered five of his children.

The painting, identified by experts as the turning point in the 88-year-old artist's career from flatness to thick brush strokes and swathes of impasto, is expected to fetch 3.5-4.5 million pounds ($4.5-7.2 million).

The work will be shown at an exhibition in Moscow on April 2 and 3 which the auctioneer said would be the first "significant" exhibition of works by Freud in Russia.

It may also be targeting wealthy Russian art collectors who, along with the Chinese, have helped drive art market prices sharply higher in recent years.

It is widely reported, but not officially confirmed, that Russian billionaire Roman Abramovich bought another work by Freud, "Benefits Supervisor Sleeping," for $33.6 million in New York in 2008, a world record auction price for a living artist.

(Reporting by Mike Collett-White, editing by Paul Casciato)