A previously unknown painting by 16th Century master Pieter Bruegel the Elder has been revealed by picture restorers, the Prado museum in Madrid says.
The painting, The Wine of St Martin's Day, shows about 100 people celebrating the first wine of the season.
Before this find, there were just 40 signed Bruegels in existence.
The museum is now negotiating to buy the painting from its owners at a price of up to 7m euros ($9.4m; £5.9m), said the El Pais newspaper.
However, the painting could fetch up to 25m euros if sold on the private market, the newspaper estimated.
"The discovery of a Bruegel is an exceptional event and not something that is likely to be repeated," said the Spanish culture minister, Angeles Gonzalez Sinde.
She refused to say how much Spain might be willing to pay for the painting, but indicated she was confident of securing it for the nation.
Bruegel the Elder painted the work between 1565 and 1568. It shows peasant women and children, beggars, thieves and drunks all struggling to get some of the wine from the first barrel of the season.
Painted in tempera on linen, it is one of his biggest known works, measuring 148cm x 270.5cm (4ft 10in x 8ft 10in).
The owners of the painting did not know it could be a Bruegel until they began to try to sell it last year.
The Prado was asked to study the deteriorated work and earlier this month an X-ray revealed fragments of Bruegel the Elder's signature at the bottom of the painting.
The Prado has just one other painting by Bruegel the Elder, entitled The Triumph of Death.