PISA has its world-famous leaning tower and Avebury has a pair of leaning stones.

But on Monday work will begin to return the Adam and Eve megaliths, which are a key feature of the 4,500-year-old stone circles, to the perpendicular.

Six years ago fencing was put up around the tilting stones, which stand 14ft 3ins and 16ft, after The National Trust became concerned for the safety of visiting tourists.

Now the trust has engaged contractors to stand the stones up straight and use a lime concrete to give them a firm foundation.

On Monday a team of archaeologists will begin to excavate the area around the stones, one of which weighs more than 50 tonnes. Then huge jacks will be moved in to gently ease them back to the perpendicular before the new concrete foundations are added.

The trust's Avebury site manager, Rob Mimmack, said: "It is good news that at long last this work can start. Many visitors have been asking when the fencing can be removed.

"The jacking process will be carried out very gently, but it will nonetheless be quite spectacular."

Work on Adam and Eve should be finished by the end of May.