The route preferred by ministers will "bury" the A303 in a tunnel as it passes the ancient monument on Salisbury Plain.

The scheme for the 6.75-mile dual carriageway improvement is the most expensive road project in southern England. But planners believe a tunnel in the area immediately next to Stonehenge will bring substantial environmental benefits.

However, there are likely to be considerable delays before the scheme begins.

Transport Minister Lord Whitty said that a public inquiry would be likely to delay the operation until 2005.

Once started, he estimated the project would take a further three years to complete.

The route improvement includes a 1.25 mile cut-and-cover tunnel, designed to take traffic "away" from the monument.

Other changes to the already-announced route have been made to take into account local environmental concerns.

The route includes a bypass of the village of Winterbourne Stoke, currently cut in two by the busy A303, and a possible flyover at the Countess roundabout near Amesbury.

But environmental groups have expressed reservations.

A spokeswoman for the Council for the Protection of Rural England said: "We are concerned about the increased traffic levels the scheme will generate and we want more details about tunnel options."