Rome’s world famous Colosseum is now around 40 cm (16 inches) lower on the south side than on the north.
Concerned authorities are investigating whether it needs urgent repairs, after experts noticed the incline about a year ago.
Rossella Rea, director at the 2,000-year-old monument, revealed in Italian daily Corriere della Sera on Sunday that officials have been monitoring it for the past few months.
The Leaning Tower of Pisa, another of the country’s most popular attractions, was reopened in 2001 after being shut for more than a decade as engineers worked to prevent it from falling over and to make it safe for visitors.
Restorers’ efforts to clean off centuries of grime from the Tuscan landmark helped stabilize the historical building by removing soil from beneath one side of its foundations.
“The slab of concrete on which the Colosseum rests, which is like a 13-metre (yard)-thick oval doughnut, may have a fracture inside it,” he told the newspaper.
He said intervention along the lines of the stabilization work carried out in Pisa could be necessary if the concerns are confirmed, but he added that it was too early to judge what kind of intervention would be most suitable.
The Colosseum – famous for hosting bloody gladiator fights in the days of the Roman Empire – attracts hundreds of thousands of tourists and is usually packed with visitors.