Italian authorities have found the south side of the 2,000-year-old amphitheater, which hosted bloody gladiator fights during the era of the Roman Empire, is 15cm lower than on the north, The Telegraph reported.
Experts from Rome's Sapienza University and the Institute of Environmental Geology and Geoengineering have been tasked with finding out the cause of the tilt.
Professor Giorgio Monti thinks there may be a crack in the base of the monument.
"The concrete foundation on which the Colosseum is built is an oval-shaped 'doughnut' which is 13 meters [42 feet] thick," Professor Monti said.
"There could be a stress fracture inside it."
According to Reuters, the Leaning Tower of Pisa was closed to tourists for more than 10 years while engineers worked to stop the structure from falling over.
The 12th century structure was reopened to the public in 2001.
The Colosseum, which was built by Emperor Vespasian in 72AD and attracts hundreds of thousands of visitors, may need similar restoration work, The Telegraph quoted Rossella Rea, the director of the Colosseum, as saying.
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