Putting a major road past Stonehenge into a tunnel could benefit the World Heritage Site if the scheme is well designed and constructed, an international report has concluded.
Heritage bodies in England have welcomed the report by the International Council on Monuments and Sites and United Nations cultural body Unesco, which said there were positive impacts of building a 1.8 mile tunnel for the A303 past the stones.
The study, made at the request of the UK Government, warned the position and design of the tunnel entrances, embankments, entry and exit ramps and construction works had the potential to adversely affect the historic landscape.
They would require rigorous investigation, evaluation, design and assessment, the report said - but with "good design and construction controls", the tunnel should have a beneficial impact on the World Heritage Site.
A scheme to tackle congestion on the A303 at Stonehenge - a notorious bottleneck on the route to the South West - including putting the road into dual carriageway tunnel past the stones, was announced by the Government in December 2014.
As part of £2 billion of works planned for the A303, the scheme - which would include the tunnel - is estimated to cost between £275 million and £1.32 billion.
Although the stones will no longer be visible to motorists driving past, the tunnel will reconnect Stonehenge with two-thirds of the World Heritage Site that lie to the south of the A303 and are currently cut off by it.
Heritage groups which manage the area have said the tunnel would make the setting of the ancient stone circle more tranquil, give the public greater access to the wider prehistoric landscape and improve the environment for wildlife.
English Heritage, the National Trust and Historic England backed the report, which pointed to the benefits of having closed the A344 next to the stones and said it showed the positive impact the removal of the A303 into a tunnel could have.
Kate Mavor, chief executive of English Heritage which looks after Stonehenge, said: "We are delighted that the mission recognises the benefits our recent improvements have brought to the World Heritage Site.
"Provided that it is designed and built in the right way, a tunnel would reunite the wider landscape around the ancient stones, helping people to better understand and enjoy them."
Helen Ghosh, director-general of the National Trust - which owns much of the land around Stonehenge, said the report recognised the "unmissable opportunity" to address the blight of the A303.
"At the moment the A303 cuts through the middle of the World Heritage Site, compromising its integrity and harming the setting of many monuments," she said.
And Duncan Wilson, chief executive of government heritage body Historic England, said: "We share the mission's view that the design and location of all aspects of the road improvement need to be very carefully considered.
"But with sensitive design there is a real opportunity both to deal with the problem of the current A303 and to deliver significant public access and landscape quality benefits to the World Heritage Site."