A Rembrandt self-portrait that lay hidden under layers of paint for hundreds of years could sell for as much as £5m when it is auctioned in July.
It took years to restore the work to its original glory
Experts identified that a much finer work had overpainted it and in 1999 it was sent to Amsterdam's famed Rijksmuseum, home of Rembrandt's masterpiece Night Watch.
Art restorers discovered that the painting had been transformed by one of Rembrandt's pupils into a study of a Russian aristocrat - a popular subject at the time.
Using X-rays, infra-red and infinite care, the museum's experts scraped away layers and layers of paint to reveal the original work.
"You have to remember that while Rembrandt was an artistic genius, he was also a commercial artist," said Alex Bell, Old Master expert for auction house Sotheby's.
"If a painting did not sell quite quickly it would be painted over into a more saleable commodity."
Mr Bell said that, as far as Sotheby's knows, the Rembrandt self-portrait is the only one still in private hands.
"It's in very good condition," he added.
Rembrandt, who is known to have painted at least 40 self-portraits, had to sell many works to other artists towards the end of his life to pay off his debts.
Rembrandt's self-portrait was transformed into a Russian aristocrat
The newly discovered image is not recorded in the literature of his works because of the overpainting.
The signed painting, which dates from 1634 when the Dutch master was at the height of his fame, is the first of his self-portraits to go on auction in three decades.
Mr Bell said the art world is already buzzing with the news that what, in effect, is a new work by Rembrandt will go on sale in London on 10 July.
A Pietro Guarneri violin that was expected to fetch between £200,000 and £250,000 as the top lot at Sotheby's auction house on Tuesday failed to reach its reserve price and remains unsold.