Alternatives to an underground road tunnel at Stonehenge could threaten the recovery of one of Britain's rarest birds, the RSPB has warned.
One third of the stone curlew population is found in the SW
The society said proposals for two overground routes would destroy nesting and roosting sites of the stone curlew, which only has two UK strongholds.
It said the plans would also harm prospects for more than 25 other bird species and 14 butterfly species.
The Highways Agency has raised its estimate of tunnelling costs to £480m.
The tunnel was given the go-ahead by the government after a public inquiry in 2004, when ministers called it an "exceptional environmental scheme".
Tony Richardson, director of the Royal Society for the Protection of Birds' south west region, said: "A completely new road through the Stonehenge site is unthinkable, not only because of its obvious archaeological value but also because of the public outcry it will spark amongst the millions of people who value Britain's wildlife.
"Approval for any overground route will make a mockery of the government's desire to get this mysterious and vulnerable bird back on its feet," he said.
The Highways Agency has raised its estimate of the 1.3-mile (2.1-km) tunnel's costs from £284m to £470m, blaming complications that will make tunnelling more difficult.
A number of alternatives are likely to be published on Monday, including the re-routing of the A303 overground instead, either north or south of Stonehenge.
Mr Richardson added: "Future generations will thank the government if it safeguards the Stonehenge area from the irreversible destruction that an overground road will cause."
In a statement English Heritage, which manage the stones, said it is reviewing the road improvement options for the A303 and will be submitting a formal response in due course.
"Until that review is completed, English Heritage remains a supporter of the published scheme and continues to believe that it would result in major benefits for the World Heritage Site and its visitors, together with users of the A303.
"For there to be any question of English Heritage changing its position, any alternative would need to offer benefits that are comparable to this scheme."