The Leaning Tower of Pisa has been saved from toppling over, according to one of the experts involved in straightening the famous landmark.

The tower is now leaning at 5.5 degrees, and the tilt has been corrected by a full 23 cm, Professor John Burland of Imperial College, London, told the British Association's Festival of Science.

This should keep the tower standing for at least another 350 years, he said, even if the tower, now steady for the first time in 800 years, started to move again.

Professor Burland is leading efforts to stabilise Pisa's famous landmark by removing soil from under the northern side of the tower.

The engineers have inserted a number of corkscrew drills at a shallow angle into the earth beneath the tower. These are being used to slowly remove 30 tonnes of subsoil.

Professor John Burland has applied the Pisa lessons to Big Ben

Tourist attraction

The weight of the tower causes it to sink into the cavity, resulting in a small corrective movement.

"As we're taking soil out, we're also removing the temporary lead weights which were placed there in 1993," said Professor Burland. "That's a very tricky operation."

Professor Burland said the soil extractions were half finished, and it was hoped to reopen the tower to tourists in June, to coincide with celebrations for the feast of Pisa's patron saint.

"If we can bring it back to where it was in 1838, before the architect Gheradesca excavated around the tower and destabilised it, then I think all the indications are that it would be safe enough to reopen," Professor Burland said. "That is what we would like, that is what the people of Pisa would like, because it would bring the tourists back, on which Pisa depends."

Big Ben project

Professor Burland revealed that a similar project had kept another famous clock tower - London's Big Ben - from wobbling during excavations for the Jubilee Line tube extension.

This project is almost the reverse of the engineering work being used to stabilise the Leaning Tower of Pisa.

Instead of soil being removed, grout - a mixture of cement and concrete - was pumped into one side of the foundations under Big Ben.

Big Ben moved by between 17 and 25 millimetres during the construction work, said Professor Burland, but was never at risk of falling over.

The clock tower wobbles naturally by nine millimetres a year as a result of seasonal factors, and has always tilted slightly to the north west, a slant that is just visible to tourists.