The Royal Society for the Protection of Birds has spoken out against plans for a £10bn Severn barrage.
The barrage would stretch 10 miles across the Bristol Channel
The UK Government is conducting an energy review looking at how the UK can meet its future energy needs.
The 10 mile-long barrage would run from Lavernock Point, near Cardiff, to Brean Down, near Weston-super-Mare.
But a spokesman for the society said the damage to the estuary as well as the wildlife in the area would mean the scheme was not viable.
He said: "The proposal to construct a 10-mile long tidal power generating barrage across the Severn Estuary is fundamentally flawed as it will have an irreversible, serious and adverse impact on the wildlife of the Severn estuary.
"The construction of, in effect, a monstrous concrete wall across one of the most dynamic estuaries in the world will damage internationally-protected wildlife sites, disrupt important fish migrations and destroy forever the character of the Severn."