A plan to generate power from Severn estuary tides would be the "wrong option", green campaigners have said.
The Severn estuary provides a habitat for thousands of birds
A report by Friends of the Earth says that the Severn Barrage would damage the environment and wildlife habitats.
The government is studying the scheme, which it is thought could produce up to 5% of the UK's electricity needs.
Meanwhile, the Sustainable Development Commission is due to release a tidal power report on Monday, including the feasibility of a Severn barrage.
It could extend from the south Wales coast near Cardiff across the estuary to Weston-super-Mare.
The idea was first raised about 150 years ago, but environmental and cost concerns have always blocked approval.
It was last seriously considered in the 1970s and 80s.
Last year's governmental energy review opened the door again, saying that the barrage should be re-evaluated.
HOW TIDAL POWER WORKS
As tide comes in, sea water passes through barrage to landward side
At high tide, sluice gates shut, trapping water in estuary or basin
When tide recedes on sea-side of barrage, sluice gates open
Water flows through barrage, driving turbines and generating power
Power can be generated in both directions, but this can affect efficiency and economics of project
Friends of the Earth's report says a system of "tidal lagoons" - artificially created offshore pools - would be better for wildlife, cost less, produce 60% more energy and not block shipping routes.
Under the group's alternative proposals, water would flow through turbines in and out of the lagoons as the tide rose and fell.
Neil Crumpton, Friends of the Earth's energy campaigner, said that this would avoid the need to destroy natural habitats in the estuary.
He said: "The £14 billion Severn barrage would be a hugely expensive, environmentally damaging and legally questionable mega-project.
"In contrast, a series of large lagoons in the Severn estuary, possibly with a Shoots barrage which could carry the London to south Wales rail link, could offer a far better solution to harnessing the enormous power of the Severn estuary."
Mudflats, saltmarshes and rocky along the estuary provide food for 65,000 birds during winter. Dr Mark Avery, the RSPB's conservation director, said: "Tackling climate change is hugely important but this can be done without destroying irreplaceable national treasures like the Severn estuary."
"The Severn estuary is one of the UK's most important sites for water birds," said the RSPB's conservation director Mark Avery.
The barrage would be 16km (10 miles) long
It would power more than 200 turbines
Planners say it could create 35,000 construction jobs and between 10,000 and 40,000 permanent jobs
The barrage could be generating electricity within 11 years, say planners
Greenpeace said it was expected that the Sustainable Development Commission's report was would recommend further analysis of the scheme.
John Sauven, executive director of Greenpeace UK, said that "the jury's still out" on the best way to harness tidal power from the estuary without affecting birds.
At the Labour Party's annual conference in Bournemouth, business and enterprise secretary John Hutton launched a feasibility study into the Severn barrage.
A spokesman the Department for Business, Enterprise and Regulatory Reform said the government was looking at all the options.
He added: "This includes both a barrage and tidal lagoons. Neither option has been ruled out.
"The Sustainable Development Commission's study has looked at all the various technologies. We will await their report and consider it carefully."