The recent sale of Rubens's masterpiece The Massacre of the Innocents for nearly �50m is a telling example of the money that can be made from rediscovering great art.

In this case, the work had been mis-attributed to Jan van den Hoecke, a follower of Rubens, until Sotheby's expert George Gordon correctly identified the painting.

While some works remain unidentified or misattributed for decades - even centuries - others lie completely forgotten in attics and barns.

Canaletto's distinctive works hung unnoticed in a conference centre

In 2000, a 13th-Century masterpiece, which for years had hung unnoticed on a landing in a country house, was finally attributed to Italian master Cimabue.

The painting of the Madonna and Child enthroned with an angel, was discovered by Richard Charlton-Jones of Sotheby's while he was cataloguing the works for a house sale at Benacre Hall in Suffolk.

Staff at the hall had no idea of the painting's significance or value.

Cenni di Pepo, called Cimabue (circa 1240-1302), was the most important figure in the development of Italian painting in the late 13th Century.

Eagle-eyed

More recently two paintings by 18th-Century Italian master Canaletto were found hanging on the wall of a conference centre in Bristol.

The conference centre owner had unearthed them in a country barn - but their true value became known only when an eagle-eyed insurer came to evaluate the centre's contents.

The Canaletto paintings of St Mark's Square in Venice house, which were hanging unprotected, have since been housed at Sotheby's.

Another art treasure - an early work by 19th-Century English artist John Constable - was found during a house valuation in Essex earlier this year.

The painting, The Daughters of Mr W Mason of Colchester, was virtually unknown to art historians.

Art expert Henry Wemyss said the only known reference to the work was in a 1937 edition of an 1840s memoir by a friend and biographer of the artist.

The painting had been passed down through generations of the Essex-based Mason family, descendants of the artist's lawyer.

The portrait is believed to have been painted in lieu of payment of a debt.

Authenticated

And one of the most spectacular finds of lost works was announced earlier this week.

Sir Timothy Clifford, director of the National Galleries of Scotland, was on sabbatical at New York's Cooper-Hewitt museum in April when he found a drawing by Michaelangelo - worth an estimated �6.5m.

The drawing, of a seven branched candle-holder in black chalk, was unsigned but was unanimously authenticated by scholars both in the US and Europe.

The drawing had survived five centuries and is now believed to be one of fewer than 10 Michelangelo works in the US.