Image caption An anonymous telephone bidder bought the Francis Bacon portrait

A Francis Bacon painting has sold for nearly £18m at auction in London on Tuesday.

Study for a Portrait 1953 exceeded expectation as Christie's had valued the masterpiece at £11m.

The oil canvas, which has not been up for auction before, was described as "an arresting and dark study in the degradation of power".

It was previously owned by two of Bacon's contemporaries - Rodrigo Moynihan and Louis Le Brocquy.

The work was the second most valuable piece to be sold at one of Christie's post-war and contemporary art sales, a spokesperson said.

The highest selling work in this category was Bacon's Triptych, which went for £26.3m in February 2008.

Other items sold at Tuesday's auction included Lucian Freud's Woman Smiling, 1958-59, which went for £4.7m. Seven works from the collection of Kay Saatchi went for a total of £3.9m.