The 2,000-year-old "House of the Gladiators" in the ruins of ancient Pompeii collapsed on Saturday, officials said.
They said the stone house, on the main street of the world-famous archaeological site and measuring several hundred square metres (yards), collapsed just after dawn while the site was closed. Custodians discovered the collapse when they opened the site.
The structure was believed to be where gladiators gathered and trained and used as a club house before going to battle in a nearby amphitheatre in the city that was destroyed by an eruption of Mount Vesuvius in 79 AD.
Known officially by its Latin name "Schola Armaturarum Juventus Pompeiani," the structure was not open to visitors but was visible from the outside as tourists walked along one of the ancient city's main streets.
Its walls were decorated with frescoes of military themes.
Officials speculated that the collapse may have been caused by heavy rains.
Art historians and residents for years have complained that the archaeological sites at Pompeii, among the world's most important, were in a state of decay and needed better maintenance.
Opposition politicians quickly criticised the government of Prime Minister Silvio Berlusconi, particularly Culture Minister Sandro Bondi, for allowing the site to degrade in recent years.