The Ankerwyke yew by the Thames near Wraysbury is some 30ft (9m) in diameter and believed to be about 2,000 years old. The yew is on the bank of the river opposite Runnymede, the meadow where King John signed the Magna Carta in 1215. Now a proposed development of 14 flats within a few hundred yards of the tree threatens to alter the special feeling of the place.
Not far from the Ankerwyke yew is Windsor Castle, residence of kings since Saxon times and near by is the site of a Benedictine nunnery, built in the 12th century. The yew may indicate that the site was sacred long before either was built.
Alan Meredith, who has studied ancient yews throughout Britain, said: 'Yew trees were used in Ireland in the eighth and ninth centuries for inauguration of chieftains - and this yew being so close to Windsor could have been used in that way.'
He has discovered that the Thames changed its course in about 1250, putting the yew on the opposite bank to Runnymede. So it is possible, and even likely, that King John met the barons near the yew, then part of Runnymede, and there signed the parchment which gave important rights and freedoms to Englishmen.
'Building houses near by will spoil this site,' said Mr Meredith. 'It has a very important place in English history. But when you get to the yew tree there is nothing to say how old it is or to explain the history of the place.'
Local people are interested in protecting the tree and getting Berkshire County Council to have a management policy for the site.
'If the tree was a building of the same age every effort would be made to preserve it,' said Mr Meredith. 'Tree preservation orders do not give much protection and cannot always be obtained.'
In Somerset, conservationists have tried to obtain orders to preserve a grove of ancient yews near Whatley, but Mendip District Council will not agree to grant preservation orders.