It was painted by a Bruegel. But which one?

The elder, it transpires – although until now it was assumed to be the work of his son

Spain's prado Museum has discovered a previously unknown work by Pieter Bruegel the Elder, the 16th-century Flemish master, after cleaning a painting that had been attributed to his less illustrious son.

Bruegel is best known today for his peaceful winter scenes but the painting, The Wine of Saint Martin's Day, certainly won't work as a Christmas card. Instead of ice skaters gliding on a frozen pond, it features peasant revellers clamouring for wine and vomiting at a grape harvest bacchanalia.

Prado curators in Madrid are excited. It turned out to be Bruegel's largest surviving canvas, and one of his few signed works. It is one of only 41 Bruegels in the world. "This was a great surprise," said Pilar Silva Marato, head of the Prado's Department of Flemish Painting at the work's unveiling yesterday. "This is one of the most important discoveries for many years with regard to his oeuvre."

According to Prado curators, the painting, nearly 3 metres wide, belongs to a prominent private collection in Spain, once owned by the 17th-century Duke of Medinaceli. The Duke is believed to have purchased it for nearly three times the cost of what he paid for The Spinners by Velazquez.

The modern-day owners of the painting believed, however, that it was executed by the less-inspired Bruegel the Younger, and they brought it to the Prado for an expert cleaning, with an option to buy. But as curators began to carefully lift away dirt, and to analyse the pigment, canvas and brushstroke, they found a surprise. "The application of the brushstroke with its deft, confident strokes, the manner of creating the folds of the drapery, the touches of light on the faces and on some of the drinking vessels, some of the shadows and the choice of colours are all characteristic of Pieter Bruegel the Elder," Ms Silva Maroto said. On 6 September, radiographs revealed the artist's signature, dated with Roman numerals between 1565 and 1568.

The Prado hopes to buy the painting, which would add an upbeat counterpoint to the museum's sole Bruegel, the apocalyptic Triumph of Death, featuring an army of skeletons.

The canvas remains under conservation, secluded in a bunker-like vault in the museum basement. Some portions have been damaged. A horse, for instance, is missing a leg.

The Wine of Saint Martin's Day is an example of the Flemish master's fascination with peasant life, as well as his Reformation-era critique of peasant excesses. The composition includes more than 100 figures – "a sort of Tower of Babel," as Ms Silva Marato put it – who spiral up a giant red wine keg at the back of a charitable Saint Martin. A pick-pocket steals. Men brawl. A mother gives her baby his first nip of wine. A drunk sprawls on the floor in his own vomit. Each figure is done in expressive strokes with subtle lights.

"He censures the customs of his times in a masterful tour de force, creating the effect of a mountain of humanity driven by gluttony," Ms Silva Marato said, comparing the composition to Bosch's The Haywain.

Bruegel the Elder was celebrated in his lifetime as "the new Bosch", but he was also known as "Bruegel the Peasant" because he would don peasant garb and crash celebrations to better study his favourite subjects. After his death, his works were hoarded by collectors. Few today are in private hands. Besides the ubiquitous Christmas card scenes, his works have also inspired poets such as W H Auden, who spun into verse Bruegel's Landscape with the Fall of Icarus.