European economic consultancy Frontier Economics has also shown that the proposal to use taxpayers money to build a £15bn dam across the estuary would not, under existing Treasury rules, warrant special government subsidies or any other form of public investment.
Frontier’s analysis, commissioned by ten UK environment groups, follows a report last October by the Sustainable Development Commission, which said if a barrage between Cardiff and Weston-super-Mare was built, it should be state funded and state run.
Those backing the scheme say it is essential to help the government hit its renewable energy generation target.
Matthew Bell, author of the report, said: 'It is hard to think of reasons for the public sector to build or operate a barrage which would not be equally applicable to many other projects and assets that sit in the private sector.
'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 generation there is.
"Frontier’s report shows that this exorbitantly expensive and massively damaging proposal cannot be justified on economic grounds."
Frontier assessed the justification for public funding of a large Severn barrage and compared its cost with the cost of generating the same amount of energy in the UK using other renewable technologies.
Variable carbon trading prices, the youth of tidal technology, the high cost of a barrage and the risks to private investors, were not sufficient grounds for state involvement in a large barrage, the study found.
The report shows the barrage to be expensive compared to other renewables and that the government’s renewables target could probably be met using cheaper green technologies.
"Considerable new evidence would be needed to make a large barrage in the Severn estuary an attractive option," Frontier say.
The RSPB, National Trust, WWF-UK, the Salmon & Trout Association and The Wildlife Trusts were amongst the groups commissioning the analysis. Friends of the Earth did not participate.