Plans to protect parts of Lincolnshire from coastal flooding have been drawn up by a private firm.
The Wash Tidal Barrier Corporation wants to build a barrier stretching from Skegness to Hunstanton in Norfolk, at a cost of more than £2bn.
The company claims the barrier would protect up to half a million people and 300,000 hectares (741 acres) of land.
But the Lincolnshire Wildlife Trust said the barrier would have a negative impact on the surrounding environment.
In a statement, the trust said: "The proposed barrier would have serious consequences for the Lincolnshire Wildlife Trust's Gibraltar Point nature reserve and the whole of the Wash - an area recognised as being of international importance for wildlife.
"Lincolnshire Wildlife Trust does not believe that a barrier would bring about benefits for biodiversity."
The proposed barrier would span the Wash from Hunstanton in Norfolk to just south of Skegness in Lincolnshire, a distance of approximately 18km (11 miles), with an additional 5km (3 miles) of barrier in Lincolnshire in order to reach high ground.
Dr Peter Dawe, founder of the private firm behind the barrier, said: "The whole basis of the barrier is to conserve, not to destroy.
"If we don't build the barrier, the sea level in the Wash will rise and rise and all those mudbanks are going to disappear and we'll just have open sea.
"When you get to the salt marshes and the Fens, what will happen is you'll have periodic salt water inundation which will be bad for freshwater and sea water environments."
The barrier plans also include a scheme to generate electricity using tidal power which Dr Dawe said "would more than pay for the barrier".
Plans are yet to be approved and permissions are being sought from the government, landowners and other interested parties but if passed, the project would be completed by 2012.