All four potential routes for the Norwich Northern Distributor Road Western Link will cause 'unnaceptable' damage to vital habitats for animals, plants and birds, wildlife watchdogs have warned.
Norfolk County Council is consulting over four possible routes to link the NDR to the A47 to the west of the city.
But Norfolk Wildlife Trust says, without a 'significant commitment' to mitigation and creating new habitats, it is likely to object to all the routes.
When the NDR, now known as the Broadland Northway, was first proposed, linking it to the A47 was rejected because of the cost of crossing the River Wensum a site of special scientific interest.
However, the county council believes a solution can be found - potentially involving viaducts.
But the wildlife trust says, apart from the river, there are other areas of importance which could be threatened.
There are several County Wildlife Sites in the area, home to the likes of voles, red kite and otters, as well as plants such as marsh bedstraw and Greater birdsfoot trefoil.
Pamela Abbott, chief executive of Norfolk Wildlife Trust, said: 'Without further evidence that the losses to important wildlife sites, degradation of nearby habitats and habitat separation across the landscape can be avoided or properly mitigated, we currently regard all of the options as unacceptable.
'The western link road will lead to direct loss of habitat, the separation of remaining habitats into smaller fragments and impacts on floodplain hydrology as well as increased light, noise, road run-off and air pollution over a considerable distance each side of the road.'
Martin Wilby, chairman of the council's environment, development and transport committee, said: 'These are important issues that Norfolk Wildlife Trust are bringing up and ones that we're very mindful of.
'We are absolutely committed to minimising potential adverse impacts and, wherever possible, enhancing the environment.
'We've already done lots of work and research into this, including commissioning our own ecologists and environmental experts and seeking advice from Natural England and the Environment Agency.'
Consultation, at www.norfolk.gov.uk/nwl, continues until Friday, January 18.
What are the options for the Western Link route?
Option A: Single carriageway upgrade to A1067 to Lenwade and B1535 to A47 Wood Lane junction at Honingham, using existing bridge at Attlebridge. Cost: £60m.
Option B: New dual carriageway and A1067 upgrade. New route to east of Weston Longville linked to A47 at Wood Lane junction. Two alternatives to link to A1067. New junction at Attlebridge, using current bridge or viaduct over Wensum, joining A1067 east of Attlebridge. Cost: £155m.
Option C: New dual carriageway and A1067 upgrade. Link to A47 at Wood Lane. New junction to take route between Weston Longville and Ringland, crossing Wensum on viaduct. Cost: £153m.
Option D: New dual carriageway and A1067 upgrade. New route west of Ringland, linked to A47 either at Taverham Road junction or near Easton roundabout junction, crossing Wensum on viaduct. Second viaduct over Tud. Cost: £161m.
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