An unsigned painting by 17th-century French master Georges de La Tour has been discovered in a Madrid mansion after hanging in offices unnoticed for decades.

The painting, "Saint Jerome Reading a Letter," is only the second work by La Tour known to exist in Spain, and one of only about a dozen of his paintings to have been identified throughout the world.

Prado director Miguel Zugaza said Friday that the find was "a cause for celebration" and that museum officials had been congratulated by experts from the Louvre museum in Paris.

"As soon as I set my eyes on it I said, 'I know you,' " said Jose Milicua, an emeritus professor of art history and member of the Prado's board. It was Milicua who certified the work as a La Tour original.

"I've been around and I've seen a lot. I knew immediately that this was a de La Tour," he said. "Of course, someone might come along and dispute it, but I'm not worried. I know what I'm talking about."

The work, unsigned like most of La Tour's works, shows an elderly, bearded Saint Jerome wearing a red cloak and using reading glasses to examine a letter. It contains La Tour's trademark use of a ray of light in the background to create sharp contrast.

Milicua said it bore a strong resemblance to another painting of the saint by La Tour, which is part of the collection of England's Queen Elizabeth II. He said the Prado's painting was of a much superior quality.

The work was spotted by Cesar Antonio Molina, director of Spain's Cervantes Institute, in the institute's offices in a late-19th-century mansion in downtown Madrid. He said "no documents in the building shed any light on where it came from, but we know it's been there for decades.

"I just saw it hanging on a wall in an office and felt it was something special, so I contacted the Prado and asked them to come and look at it."

The painting was taken to the Prado in March and studied by experts. The museum also consulted former Louvre director and La Tour expert Pierre Rosenberg, who confirmed the find.

Spain's only other La Tour is a painting titled "Blind Hurdy-Gurdy Player," which the Prado bought in 1991.

Born in Vic-sur-Seille in northeastern France, La Tour was a renowned painter of religious subjects, best known for his dramatic use of light effects in night scenes and his simplified form of painting. He was a court painter to Louis XIII.

Besides having many of his works destroyed during a sacking of the French town of Luneville in 1638, La Tour was quickly forgotten after his death in 1652. His name first began to resurface in the early 1900s when German expert Hermann Voss began uncovering his works.