Government advisors have defended their opposition to the Norwich Western Link - warning it would destroy one of the UK's largest surviving barbastelle bat populations.
Natural England said it had advised Norfolk County Council about the risk to the protected bats on the route for more than four years, while plans were drawn up.
And, as consultation over the plans drew to an end, the organisation lodged its official objection to the council's plans for the 3.9-mile-road.
Experts said they do not believe mitigation measures - such as green bridges and underpasses - will prevent harm to bats.
James Marshall, Natural England deputy director for Norfolk and Suffolk, said: "Natural England is clear that development and nature can go hand in hand and are committed to making that happen.
"But that includes ensuring our most precious wildlife is protected from irreversible harm.
"Barbastelle bats are incredibly rare and are found only in mature woodland that can’t be recreated elsewhere.
"The proposed route for the Norwich Western Link road would destroy one of the largest remaining populations in the UK."
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Graham Plant, the Conservative-controlled council's cabinet member for highways, infrastructure and transport, said the objection was "disappointing".
He said the authority needs to consider Natural England's comments, but reiterated how the road is needed to improve safety, cut journey times and stop rat-running.
The route of the £274m road, which would connect the Northern Distributor Road to the A47 near Honingham, was previously altered to avoid habitats used by bats.
But Mr Marshall said: "We cannot see how the proposed road would not destroy the bats' habitat, even if there was mitigation in place, so we have objected to this planning application, which will also risk harm to important wildlife sites.
"We have worked closely with Norfolk County Council for more than four years on the proposed Norwich Western Link road and consistently advised on our significant concerns.
"It is now for the county council to decide whether to grant planning permission."
County Hall has applied to its own planning committee for permission for the road.
But there is pressure for the matter to be 'called in' for consideration by a planning inspector, which could leave the final decision in the hands of government ministers.
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