The traces were collected using tiny probes introduced into a wall covering the original surface in a lavish hall in the Palazzo Vecchio and contained a black pigment also used in the "Mona Lisa," historians and officials said.

The research is the result of a decades-long quest using cutting-edge technology by University of California San Diego professor Maurizio Seracini, who was featured in Dan Brown's bestselling novel "The Da Vinci Code".

"The composition of manganese and iron found in the black pigment has been identified exclusively on Leonardo's paintings," Seracini, whose methods have sometimes stirred art world controversy, told reporters in the Italian city.

Seracini also said that Leonardo had painted the "Mona Lisa" at around the same time as the long-lost "Battle of Anghiari" in the 16th century but said more research was needed to unlock one of art history's greatest mysteries.

The probes also discovered red lacquer and brown pigment on the hidden wall, which researchers said indicated the wall had had a fresco painted on it.

The experts pointed to documentary evidence from the period showing that only Leonardo could have been the author of any work on the older wall.

The probes found an air gap of around three centimeters in some places between the old wall and the new wall built in front of it.

Da Vinci (1452-1519) began his painting of the 1440 battle between Milanese and Florentine forces in a vast hall in Florence's traditional seat of government in 1505 but never finished it because the colors began to run.

The fresco was nevertheless praised by Da Vinci's contemporaries for what art historian and fellow painter Giorgio Vasari called its "graceful beauty"

and Flemish Baroque painter Peter Paul Rubens drew a famous copy of it.

Renaissance master Benvenuto Cellini said it was "the school of the world."

The Rubens sketch shows a bloody scene of horsemen battling with swords drawn and trampling over infantry men -- their faces contorted with rage and their muscled horses entwined with eyes bulging out with fear.

Da Vinci was a Renaissance polymath and the author of what has become the most famous painting in the world, the "Mona Lisa". But very few of his works survive and there are frequent attempts to find traces of his documented work.

Some historians believe Vasari built a wall in front of the fresco so as to preserve Da Vinci's efforts out of respect for the renowned master and then painted his own work, "The Battle of Marciano", on the new wall in 1563.

Seracini said Vasari himself left a tantalising clue on his painting about the hidden Leonardo with an inscription on a banner held up by one of the soldiers in the battle that reads "Cerca Trova" ("Seek and You Shall Find").

The research has been partly funded by National Geographic and the US group's vice president Terry Garcia said: "I am convinced that it is there."

Florence mayor Matteo Renzi said: "This is very exciting, very emotional and very important for the future of our city.

"This is not a crusade by some crazy guy in love with some mystery but a crucial issue for cultural policy in our country," he said.

Renzi said he had also asked the Italian government for permission to carry out further probes through the Vasari painting in over a dozen areas where the original paint no longer exists and has been touched up over the centuries.

Asked whether public funding would be needed for further research, he said: "We're talking about Leonardo here. The money will come from around the world."

The research in Florence has been controversial however and has been even investigated by art police because the researchers had to bore six small holes into Vasari's work out of the 14 they had requested to reach the hidden wall.

International art scholars and the Italian heritage group "Italia Nostra"

last year signed a petition complaining that the search was nothing more than a "Dan-Brown style" publicity stunt which risked damaging Vasari's fresco.

Art historian Tomaso Montanari, who teaches at the University of Naples, said in his blog that the search for the "Battle of Anghiari" was "tragi-comic"

adding: "It will not be found but what counts is the mediatic effect."

Seracini said the criticism had "slowed down" and "damaged" the project. (AFP)