International advisers have objected to plans to put a major road into a tunnel by Stonehenge because of "negative and irreversible" impacts on the World Heritage Site.
Highways England has proposed turning the A303 into a 1.8-mile (2.9km) dual carriageway tunnel where it passes Stonehenge to cut congestion and improve the surroundings of the standing stones.
Under the plans, the tunnel entrances would still be in the World Heritage Site, with the western end emerging close to the Normanton Down Barrow group, one of the key monuments.
But the International Council on Monuments and Sites (Icomos) UK, which advises United Nations cultural body Unesco, said it "firmly objected" to the proposals.
Important criteria had not been met, including ensuring the tunnel was long enough that its entrances did not harm the World Heritage Site and adequately considering options for constructing a bypass outside the 10 square mile protected area.
And the proposals went against the Government's own planning policy which said that substantial harm or loss of designated heritage including World Heritage Sites should be "wholly exceptional", Icomos said.
In a submission to Highways England seen by the Press Association, Icomos said it approved in principle of the idea of a tunnel.
The focus was on the visual and acoustic improvements for the famous stone circle and the tunnel's entrances would not be visible from there, it noted.
But the area received World Heritage designation not just for the standing stones but for all the monuments and archaeological features - known and unknown - that make up the wider Stonehenge and Avebury landscapes.
The western portal and 1.2 miles (2km) of new dual carriageway in previously undeveloped land within the site has the potential for "irreversible damage" to highly significant monuments, Icomos said.
Benefits to parts of a World Heritage Site from the road development could not mitigate damage to other parts of it, the heritage experts warned.
The consultation's technical assessment was missing "archaeological details on even a very basic level" and lacked analysis or appreciation of irreversible impacts on archaeology and features such as solstice alignments.
Protecting the World Heritage Site had been given a low priority in the overall assessment process, below that of areas of outstanding natural beauty and other concerns.
And the length of the tunnel had been decided on cost not heritage grounds, Icomos said.
Buckingham University archaeologist Professor David Jacques, who has opposed the plan because they would damage archaeological research inside the heritage site's boundaries, said the report was the "death knell" for the tunnel.
He said: "The Stonehenge landscape is providing people with the earliest British stories, that go back evidentially to just after the Ice Age.
"Stonehenge is part of all our history, it should be studied in perpetuity, our children and grandchildren should be allowed to research it.
"The tunnel, if it had gone ahead, would have slammed the door shut on these wider research avenues."
A Highways England spokesman said: "Consultation gave everyone the chance to have their say on the proposed options and Highways England will carefully read every piece of feedback from all interested parties.
"We will continue to work closely with key organisations within the World Heritage site, as we continue to find the best solution possible to protect Stonehenge, improve journeys for drivers and help address rat running on local roads."