"La Surprise", depicting a musician tuning his guitar as he sits next to an amorous couple, was considered by its owners to be a copy until the auctioneer spotted it when called in to evaluate the contents of a British country house.

Art experts had only known about the painting, made in around 1718, by a copy in the Royal Collection at Buckingham Palace and through a contemporary engraving.

"It was extremely exciting to have rediscovered the painting last year, its whereabouts having been a mystery for almost 200 years," said Richard Knight, international director of Christie's old master department.

Earlier in the day, three sketches by Spanish master Goya, also presumed lost for more than 130 years, went under the hammer for $7.9 million, double the pre-sale estimate.

The drawings, also auctioned by Christie's, were last recorded at a Paris sale of works by the artist in 1877 and all come from Goya's celebrated private albums.

They were sold from a Swiss private collection and were in exceptional condition because they were never framed or exposed to light.

The top lot of the three was "Down They Come," from album D called "Witches and Women," depicting four women fighting as they fly through the air. It fetched $4.5 million, a world record at auction for a Goya work on paper.

Next was "Repentance," representing a seated man praying before a cross, which raised $1.9 million, and finally "The Constable Lampinos Stitched Inside a Dead Horse" ($1.5 million).

Goya outlines the story of Lampinos at the bottom of the drawing, explaining how an unpopular and corrupt official was stitched inside a dead horse by local people as punishment.