Campaigners against the Norwich Western Link, including naturalist Chris Packham and broadcaster Stephen Fry, have written to the leader of Norfolk County Council urging him to scrap the £251m road.
The "grave concerns" of representatives from a string of organisations, including the Norfolk Wildlife Trust, the Campaign to Protect Rural England, the RSPB, The Woodland Trust and Norfolk Rivers Trust, have been outlined in a letter to County Hall leader Andrew Proctor.
The intervention comes as the Conservative-controlled council launched a public consultation ahead of the submission of plans for the 3.9 mile road, which includes a viaduct over the River Wensum, early next year.
The signatories to the letter say the road, which would connect the Northern Distributor Road to the A47 near Honingham, will "fragment" a wildlife-rich area.
They say it will damage the River Wensum Site of Special of Scientific Interest and Special Area of Conservation and destroy parts of three County Wildlife Sites.
They say: "The area is home to ancient woodlands, grasslands, chalk streams and floodplains and supports ancient oak trees and species including bats, badgers, farmland birds such as yellowhammer and linnet, and a wide variety of invertebrates."
They are particularly worried at the threat to the protected barbastelle bat species.
The council has altered the alignment of the road and said mitigation, including green bridges and underpasses, will protect the habitat of those bats.
But those who have signed the letter, including members of the Stop the Wensum Link group, Norfolk and Norwich Naturalists Society and Norwich Friends of the Earth, say there is no way of building the road which would protect the bats.
The letter states: "Of most concern is the evidence from a growing body of independent research, which presents incontrovertible evidence that the proposed Norwich Western Link route goes through the heart of the UK’s largest known barbastelle bat colony.
"Barbastelle bats are in serious decline globally and the proposed road will separate them from the fields and marshes where they feed and inevitably result in the deaths of many bats. This risks the local extinction of a globally threatened species.
"We consider this to be totally unacceptable and reject any claims the council makes that they can build the road and safeguard the bat population.
"Barbastelle bats have the highest level of legal protection, a colony of this size should be designated and protected from all forms of development."
The county council is still waiting to find out if the government will provide £213.4m towards the cost of the road.
The council says, at the northern end of the route, two options for a wildlife crossing are under consideration - another green bridge or a ‘landscaped bat crossing’.
The council says existing trees could be retained either side of the road and in a central reservation, where the carriageways would separate, to guide bats to fly over the road above the tree canopy.
Martin Wilby, the council's cabinet member for highways, transport and infrastructure, said: “The details we have shared this week demonstrate we are taking a great deal of care to balance the need to protect the local environment while providing a vital link road that will make an enormous positive difference to the people living and working in our county.
"Without the Norwich Western Link, the traffic congestion and delays being experienced daily on roads and in communities to the west of the city will get significantly worse as the population of Greater Norwich grows.
“We are taking an evidence-based approach to this project and our proposals are being informed by advice from Natural England and ecologists, including bat experts."
BBC Springwatch presenter Chris Packham and Norfolk-educated Mr Fry had signed a previous letter criticising the road.
How to have your say
People can view the proposals and give feedback via an online questionnaire on the Norfolk County Council website at norfolk.gov.uk/nwl.
Hard copies of the consultation brochure and questionnaire can also be requested.
The public consultation will close on Sunday, October 9.
Four consultation events will be held at:
- Barnham Broom Village Hall, from noon to 8pm on Friday, September 2.
- The Costessey Centre, from 1pm to 8pm on Friday, September 9.
- Weston Longville Hall for All from noon until 8pm on Thursday, September 15.
- Felthorpe Village Hall from noon until 8pm on Thursday, September 22.
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