Art treasures either presumed lost or previously dismissed as copies are the focus of a series of old masters auctions in London next week.

Sotheby's and Christie's once again go head to head, having recently broken a series of records with their main summer sales of impressionist, modern, post-war and contemporary art that fetched over $1 billion (500 million pounds).

While the old masters sales tend to be smaller, with a more limited pool of works available, the world's two leading auctioneers are offering art worth a combined total of more than $120 million on Tuesday, Wednesday and Thursday.

One of the top lots at Sotheby's is "Portrait of Willem van Heythuysen" by Frans Hals, one of the leading painters of the Dutch Golden Age.

For about 300 years after it was painted in the 1630s, the work was considered an authentic Hals, but was demoted to a copy of another version of the portrait now in a Brussels gallery.

According to the auction house, it disappeared after 1963 before being recently re-discovered and re-instated as a genuine Hals. It is expected to make 3-5 million pounds.

"The most exciting things are very often the re-discovered ones," said Alex Bell, head of Sotheby's old master paintings in London. He added that the thriving market for other sectors was boosting interest in old masters.

"I've felt more interest in this sale than for a while," he told Reuters.

The company is also offering a painting by master JMW Turner, one of only a handful of major works by the artist in private hands with a pre-sale estimate of $10-14 million.

Paintings from the collection of the late eccentric German philanthropist Gustav Rau, who ran a hospital in Africa from where he would fly to London to attend auctions, are also expected to fetch a combined $4.2-6.6 million.

WATTEAU AND GOYA

At Christie's, a work by French painter Jean-Antoine Watteau has a pre-sale estimate of $6-10 million.

"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.

Christie's called it "one of the greatest paintings by the artist to have been seen on the market for decades."

The company is also selling three drawings by Spain's Goya last recorded at an auction in Paris in 1877 and presumed lost until they appeared in a Swiss private collection.

The works, taken from his private albums, date from the early 19th century and are titled "The constable Lampinos stitched inside a dead horse", "Down they come" and "Repentance".

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

According to the inscription, he survived the night, but in a subsequent drawing by Goya, now at the Metropolitan Museum in New York, he was eventually killed when they pinned him down and injected with lime through a giant syringe.

The three drawings are expected to sell for more than $4 million altogether.