"Grave concerns" about damage the Norwich Western Link would do to rare bats has prompted a wildlife charity to urge the government to hang fire on awarding millions of pounds for the controversial road.
Ministers recently raised the prospect that the 3.9-mile road, connecting the Northern Distributor Road to the A47 west of Norwich, could get money 'saved' by scrapping part of the HS2 rail route.
But Norfolk Wildlife Trust has written to the Department for Transport urging no decision over bankrolling the £251m road until "clear evidence" that rare and legally protected barbastelle bats would not be endangered by Norfolk County Council's plans.
The route, which includes a viaduct over the Wensum Valley, has already been altered because of a super-colony of bats.
The Conservative-controlled county council has promised mitigation measures, such as green bridges and underpasses.
But Eliot Lyne, chief executive of Norfolk Wildlife Trust, said: "The impacts of such a scheme on these sensitive and irreplaceable habitats and their biodiversity cannot be effectively mitigated.
"In our professional view, the road scheme is unlikely to get planning permission due to the mounting and detailed long-term scientific evidence that it will irreparably harm the UK’s largest population of rare and threatened barbastelle bats.
“We, therefore, request there be no decision on any further public funding for this road until clear evidence is presented that Norfolk County Council’s proposed mitigation would be effective for barbastelle bats, to ensure public money isn’t wasted on a project that would never pass planning consent."
Graham Plant, the county council's cabinet member for highways, transport and infrastructure, said: “We’re taking our environmental responsibilities very seriously.
"Extensive ecology surveys over several years. along with expertise and advice from bat specialists are informing our proposals.
"We will need to demonstrate through the planning process that our proposals to mitigate potential impacts and support local bat populations are sound, and everyone will have the opportunity to view and comment on these proposals once the planning application has been submitted."
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