Four years after it was last cleaned and repaired Michelangelo's statue of David in Florence is "at risk of collapse", according to a restoration expert.

Antonio Borri, professor of construction engineering at Perugia University and part of the team monitoring the statue's state of conservation, said that cracks which been repaired during a 2004 restoration marking the 500th anniversary of the statue's creation had re-appeared.

A seminar in Florence tomorrow will discuss the options for saving the statue, which is kept at the Galleria dell Accademia and attracts more than a million people a year. These include enclosing it in a protective covering to stop further deterioration and even closing it to the public altogether for a period.

Professor Borri, who is a Florentine, said that the cracks had "re-opened one by one. David is coming apart". He said the blame lay with traffic vibrations and the pressure of thousands of daily visitors. Michelangelo's masterpiece — held by many to be the most perfect representation of the nude male form ever sculpted — was also vulnerable because of its huge size and the poor quality of marble Michelangelo used, Professor Borri said.

He said that the statue was under "round the clock observation", and seismic monitors would be inserted under the statue's base to measure the vibrations. "We have got to do something quickly," he said. However Cristina Acidini, head of museums in Florence, played down the alarm.

Professor Acidini said: "We are evaluating what measures to take to protect the statue in view of its known fragility", but there was "no cause for immediate concern". The statue was being constantly monitored, and the only danger of collapse would be if Florence was struck by an unusually powerful earthquake. "But in that case the museum itself would be at risk, together with much of Florence's artistic heritage."

The 2004 restoration involved removing grime and sulphate deposits using distilled water and cellulose as well as repairing cracks. This drew criticism from art lovers who feared that the statue could be damaged during cleaning. The restoration revealed that the statue, which weighs six tonnes and is 16ft high, was showing signs of stress around the ankles.

The statue of David was commissioned by the Florence guild of wool merchants in 1501 and spent centuries in Piazza della Signoria outside the town hall — formerly the palace of the ruling Medici family — open to the elements. It was damaged during a riot in 1527, and in 1843 was cleaned using chloric acid.

It was moved to the Galleria dell Accademia in 1873, where it was better protected, although in 1991 a deranged Italian painter smashed a toe on its left foot with a hammer.

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