Britain plans to bore a road tunnel near the prehistoric Stonehenge monument to ease congestion and remove most vehicles from view of the ancient stone circle that has puzzled visitors and scholars for centuries.

The single-carriageway A303 road in southwest England that runs alongside the monument has long been plagued by traffic jams.

As part of a 15 billion pound ($23 billion) plan unveiled on Monday to upgrade Britain's road system and remove some of the worst black spots, the tunnel will take traffic underground near the World Heritage Site.

"This is the probably the biggest change to the environment around Stonehenge since the Stone Age because clearly when it was constructed 3,000 years ago it wasn't planned to be right next to a very, very busy and over-congested A-road," said Deputy Prime Minister Nick Clegg.

No one knows for sure either how ancient Britons got the stones, which weigh up to 45 tonnes, to the site or what they used them for. The stones may have been a temple, a burial ground, an astronomical calendar or all three, scholars say.

No date has been set for tunnelling to begin. – Reuters