The 2,000-year-old remains in the area of Rome's Palatine Hill, where emperors once built lavish residences, are becoming unstable and pose a risk to the 4 million tourists who visit each year.

Years of poor preservation have left many buildings in the open-air forum in a state of erosion, with heavy rains allowed to undermine foundations. There are crumbling galleries, cracked walls and unstable blocks of stone.

"We have a sick patient with many diseases," said Angelo Bottini, the head of Rome's archaeological office. "We need to find out which ones are the most serious and intervene."

The alarm was raised in November when a 15-metre stretch of wall, which experts had considered solid, fell. Italy's culture ministry ordered a survey of all the ruins, but Mr Bottini said that, for the longer term, a proper preservation scheme was needed.

The estimate for a full restoration is €130m (£90m) over 10 years. The Palatine gains an annual revenue from ticket sales of about €25m. Italy has earmarked €60m over the next 15 years for all of the country's ancient sites.

Two Roman emperors' houses on the Palatine Hill are safe for the foreseeable future, Culture Minister Rocco Buttiglione said on Thursday .

Presenting a five-month study of the Palatine's state of repair, the minister said it would take "four to five" years to make the residences of the emperors Augustus and Tiberius secure .

He said there was no risk of the sites being closed like another of Rome's biggest tourist draws, Nero's Domus Aurea (Golden House), which shut its doors for two years because of security fears in December .

On the contrary, a number of new Palatine sites are set to open, the minister said without identifying them .

Work is already under way to ward off the risk of collapse at the two imperial houses, Buttiglione said .

"Prevention is better than cure," he said. The most immediately important work will cost about 15 milion euros while a ten-year plan to make the whole of the hill safe has a bill of 130 million euros .

Buttiglione said current budget allocations would not pay for this, suggesting that some of the sites on the Hill - which bears the ruins of Imperial Rome's most important dwellings - should be subject to an extra entrance fee .

However, he added, less well-off visitors should be exempted from any extra fees .

"I want to give the ticket system a comprehensive review," Buttiglione said .

The Palatine draws more visitors than any other ancient site in Italy, over four million a year (of whom three million pay entrance fees) .

Visits have risen by 12 percent over the last four years .