The latest plans for the Norwich Western Link "do nothing" to address damage to an under-threat bat species, a Norfolk charity has said.
The Norfolk Wildlife Trust and a bat expert say they have "grave concern" the revised route - which is intended to avoid roosts - will still be "catastrophic" for the rare barbastelle bat.
On Monday, Norfolk County Council agreed to continue pursuing the 3.9mile road which will stretch from the A1067 Fakenham Road to the A47 at Honingham, including a viaduct over the River Wensum.
While critics argue the changes will not go far enough, County Hall said it has planned to mitigate any issue.
Dr Charlotte Packman, an independent expert, has been carrying out research into the presence of barbastelle bats on the proposed route.
To support the plans, County Hall has produced its own surveys, which Dr Packman described as "limited" with “colony counts, home ranges, foraging areas and roosts substantially underrepresented".
She said: “At one colony the peak count recorded by the council’s surveys is reported as 16 barbastelles, where we have recorded 105.
“Our data has been verified by UEA, making this currently the largest known barbastelle roost in the country.
“We have an important obligation to ensure that it is properly protected before it is too late."
Mike Jones, Norfolk Wildlife Trust’s conservation officer said the revised plans avoid ancient trees but do not address other concerns.
“Barbastelle bats are in serious decline globally and here in Norfolk, we are the guardians of one of Europe’s most significant maternity colonies.
“The survival of these bats relies on us considering the long-term impacts to the species on a landscape scale.”
In particular, Mr Jones said the road would run through the heart of the bat's foraging grounds, which will lead to deaths and risk the extinction of a globally threatened and legally protected species.
Martin Wilby, Cabinet Member for Highways, Transport and Infrastructure at Norfolk County Council said the authority’s bat surveys have been extensive and have been used to inform the design of the road and mitigation measures.
He said: “We still have work to do and further information on how we will reduce and mitigate environmental impacts will be available in our upcoming consultation and in our planning application."
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