The chief executive of the Wildfowl and Wetlands Trust at Slimbridge has called for a full inquiry into the impact of a possible barrage across the Severn.
Martin Sprey from WWT is "very concerned" about its potential impact on the globally important salt marsh.
He said a barrage would change the tidal system "quite substantially".
"I'm asking the government to run a full environmental impact assessment on both the barrage and the impact on the coast of south Wales," he said.
The on-going proposal for an energy generator on the River Severn was discussed in detail by Gloucestershire County Council last month