Leonardo's lost masterpiece would be a new (and secular) Last Supper

If any painting survives behind this Florence fresco, it is almost certainly The Battle of Anghiari

Banner showing the painting which might be hidden behind the wall in Florence's Palazzo Vecchio

Banner showing the painting which might be hidden behind the Vasari wall in Florence's Palazzo Vecchio. Photograph: Dario Thuburn/AFP/Getty Images

Leonardo da Vinci's painting The Battle of Anghiari was his greatest work: a fearsome, disturbing vision of war. It was the only time he got commissioned to do a painting whose dark, dissonant theme allowed him to translate the strange images of grotesque faces and machines of war that proliferate in his notebooks into the grandeur of a mural. Even though it vanished from sight centuries ago, it still has the power to haunt and fascinate.

After decades of searching, the art investigator Maurizio Seracini and his team have apparently found a cavity behind one of six huge history paintings by Giorgio Vasari that today dominate the great hall of the Palazzo Vecchio in the heart of Florence, where Leonardo left his battle unfinished in 1506. On a deeper layer of the wall protected by the cavitythey say there are traces of paint, including a black used by Leonardo in the Mona Lisa.

This is incredibly exciting, for the simple reason that if any wall painting survives behind Vasari's works it is, almost certainly, The Battle of Anghiari. It is hard to imagine what other picture it could be. So if these fragments of colour turn out to prove the existence of a painting, that painting is probably Leonardo's lost work.

Sceptics have been wrong-footed. Art historian Tomaso Montanari, who led a campaign against this intrusive research, has been quoted as saying "anything from that era could be painted on that wall". What does he mean? Who else apart from Leonardo decorated this room? No other wall painting is recorded here before Vasari did his repaint job.

This hall was built in the 1490s at the behest of the preacher Savonarola. It was not honoured with any frescoes until Leonardo was commissioned in 1503 to begin his battle picture. After he left it unfinished, no other wall paintings are recorded until the middle of the 16th century, when Vasari, court artist to Cosimo I de' Medici, remade the room.

Even the colours that have apparently been found – black, brown, and a red laqueur – fit what is known of Leonardo's battle painting. If it is him, the traces will not be of proper fresco, for Leonardo didn't do fresco – he experimented with a method more like oil painting. For that and other reasons The Battle of Anghiari is not likely to be in great condition.

But a Leonardo is a Leonardo. This one exerts a terrific pull on the imagination even though we can only look at tiny preparatory drawings. The original would be a new (and secular) Last Supper. This might just be the most important hole in a wall since the one Howard Carter looked through to see the treasures of King Tut.