The two 'Cove' stones at the centre of the Northern Inner Circle at Avebury have been leaning precariously for at least 300 years.

The Cove stones

Since 1997, on advice from the National Trust's engineers, they have been fenced off in case one of them falls without warning.

After careful analysis of the options available, the Trust, with support from English Heritage, has appointed a contractor to return the megaliths to a vertical position.

A scaffolding framework is now being erected to support the giant jacks which will be used to adjust the angle of the stones, one of which weighs at least 50 tonnes.

The jack with blocks of stone to act as a counterbalance.

One stone is about 4.9 metres (16 feet) high, the other 4.4 metres (14 feet 3 inches).

Before the operation starts, a team of archaeologists will excavate the area around the stones.

Once their work is done the contract will go ahead and should be completed by the end of May.

The fencing will then be removed, allowing public access to the stones again for the first time in six years.

Work will continue until the end of May 2003.

Robert Mimmack, the National Trust's Property Manager at Avebury, said: "It is good news that at long last this work can start.

Many visitors have been asking when the fencing can be removed and we can now give them a definite answer.

The jacking process will be carried out very gently to avoid damage to the stones, but it will nonetheless be quite spectacular."