It is a national icon, one of the world’s greatest tourist attractions – and an appalling advertisement for Italian builders. But now, 18 years after it was closed to the public for fears that it might topple over, the Leaning Tower of Pisa has been stabilised and has been declared safe for at least another three centuries.

“All of our best expectations have been confirmed,” said Michele Jamiolkowski, emeritus professor of geotechnical engineering at Turin Polytechnic, who led the project to safeguard the tower. “Now we can say that the tower can rest easy for at least 300 years.”

If Professor Jamiolkowski is right – and William Hill yesterday was giving odds of 4-6 that he is – the Committee to Safeguard the Tower of Pisa will have succeeded where many others have failed. Its medieval architects had got only as far as the third floor when the structure, built on unstable clay, began to lean in 1178. Benito Mussolini ordered it to be returned to a vertical position in the 1930s, with concrete poured into its foundations, but the result was that the tower sank further.

By 1990 it had tilted more than four metres off its true vertical, and conservationists estimated that the entire 14,500-tonne structure would have collapsed “some time between 2030 and 2040”.

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The tower reopened at the end of 2001, after a €30 million operation in which it was anchored to cables and lead counterweights while 70 tonnes of soil was removed from the north side – away from the lean – and cement was injected into the ground to relieve the pressure.

The team stabilising the tower was advised on excavation techniques by John Burland, Professor of Soil Mechanics at Imperial College, London, who also played a key role in securing the tower of Big Ben during construction of the Jubilee Line extension at Westminster.

The tilt has now returned to where it was in the early 19th century, with a lean out of true of 3.99 metres, and the tower is being cleaned by restorers using lightweight scaffolding.

Professor Jamiolkowski said that there had never been any intention to straighten the 56m tower, only to stop it sinking further. The Pisa authorities are planning to reopen a “secret” side door in the tower, closed in the 1930s, enabling visitors to look up to the sky through the tower’s eight storeys. The view had been obstructed by instruments monitoring the structure’s stability, which will now be removed.

A ceremony marking the completion of the final phase of restoration will be held at the end of the summer, according to Giuseppe Bentivoglio, technical director of the site.

The Leaning Tower is the campanile, or belltower, of the cathedral and as constructed in three stages, starting in 1173 at the height of Pisa’s military and economic power, and completed two centuries later.

The seventh floor was completed in 1319 and the bell chamber in 1372. Galileo Galilei is said to have dropped cannon balls from the tower in a gravity experiment.

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