The power generated by the proposed Severn Barrage could be produced more cheaply using other green technologies, a report says.
The £15bn dam across the Severn estuary from Cardiff to Weston-Super-Mare in Somerset could supply 5% of the UK's electricity within 14 years.
But an independent report commissioned by 10 environmental groups said it was not a good use of taxpayers' money.
Campaigners have also spelt out the damage to wildlife it could cause.
A feasibility study by the Welsh Assembly Government and the UK Government into the barrage, which could stretch from Lavernock Point, near Cardiff, to Brean Down, near Weston-Super-Mare, was announced in January.
It would harness the tidal power of the estuary using a hydro-electric dam, but filled by the incoming tide rather than by water flowing downstream.
"Considerable new evidence would be needed to make a large barrage in the Severn estuary an attractive option"
The government previously said the scale of the 10-mile barrage and the impact it could have on securing energy supplies and tackling climate change was "breathtaking".
But a report published by Frontier Economics, an economic consultancy, said the barrage would be an expensive option compared to other renewable energy and the government's renewable energy target could probably be met using cheaper green technologies.
It said: "Considerable new evidence would be needed to make a large barrage in the Severn estuary an attractive option."
The research comes after a report in October by the Sustainable Development Commission which said if the barrage was built, it should be state-funded and state-run.
But Frontier said under existing Treasury rules this would mean it would not be eligible for special government subsidies or public investment.
Matthew Bell, author of the report, said: "Not only is the private sector more than able to finance a scheme of this scale but, even using the most conservative estimates of costs, the barrage is one of the most expensive options for clean energy there is."
It is not just the economic cost that environmentalists are concerned about.
They have also warned of the "ecological destruction" a barrage could bring.
Campaigners say some 14,000 hectares of saltmarsh and mudflats would be lost through the building of a large barrage, resulting in the loss of migratory birds that nest there.
It would also hit the fish populations of the Severn, Wye and Usk rivers, which all flow into the estuary above the point where the dam would be built.
Mark Lloyd, director of the Anglers' Conservation Association, said salmon in particular would be lost.
"The salmon population of the Wye and Usk is very important in maintaining a species but also economically, the Wye and Usk rely really heavily on salmon fishing for income," he said.
He added migratory fish would be playing "Russian roulette" with the barrage's turbines at every tide.
The Frontier Economics report was commissioned by the Anglers' Conservation Association, RSPB, Salmon & Trout Association, The National Trust, The Wildfowl and Wetlands Trust, The Wildlife Trusts, United Usk Fisherman's Association, WWF-UK, Wye Salmon Fishery Owners Group, Wye and Usk Foundation.
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