A 10-mile barrage across the Severn is among five projects on a shortlist for potential schemes to harness the tidal power of the estuary published by the Government today.
Two innovative "lagoon" schemes, which would impound a section of the estuary without damming it, and two smaller barrages are also on the list.
Publishing the proposed shortlist today, Energy and Climate Change Secretary Ed Miliband said ministers had not "lost sight" of other innovative plans, including a huge "reef" project and tidal fences, which had been on a list of 10 schemes under consideration.
He announced £500,000 of funding to develop the new technologies such as the tidal reefs, which supporters say could harness the power of the estuary without causing the environmental damage associated with a barrage.
And he said progress on those technologies would be considered before any final decisions on a tidal power scheme for the Severn estuary were made.
The proposed shortlist, which is now being put out to public consultation, is as follows:
* The Cardiff-Weston barrage - a 10-mile scheme stretching from near Cardiff to near Weston-super-Mare which could generate up to 5 per cent of the UK's energy needs;
* Shoots barrage - a scheme further upstream which would generate around 1GW, equivalent to a large fossil fuel plant;
* Beachley barrage - an even smaller scheme, just above the Wye River, which would generate around 625MW;
* Bridgwater Bay lagoon - a proposal which would impound a section of the estuary on the coast between east of Hinkley Point and Weston-super-Mare, which could generate 1.36GW;
* Fleming lagoon - a similar scheme which would generate the same amount of power from a section of the Welsh shore between Newport and the Severn road crossings.
The Severn, which has the second-largest tidal range in the world, has the capacity to provide significant amounts of "green" electricity but conservationists fear some of the plans for the estuary could be hugely damaging to wildlife.
For example, the Cardiff-Weston barrage could destroy between 11,000 and 15,000 hectares of saltmarsh and mudflats, which under European law would have to be replaced with compensatory habitat elsewhere at an estimated cost of £1 billion to £3 billion.
Mr Miliband said there were tough choices to be made in fighting climate change, the "biggest long-term challenge we face".
"Failing to act on climate change could see catastrophic effects on the environment and its wildlife, but the estuary itself is a protected environment, home to vulnerable species including birds and fish," he said.
"We need to think about how to balance the value of this unique natural environment against the long-term threat of global climate change."
He went on: "The five schemes shortlisted today are what we believe can be feasible, but this doesn't mean we have lost sight of others.
"Half a million pounds of new funding will go some way to developing technologies still in their infancy, like tidal reefs and fences.
"We will consider the progress of this work before any final decisions are taken."
A tidal fence project over part of the Cardiff-Weston line with tidal stream turbines to harness the ebbing and flowing tides, and a reef proposal on the line of the outer barrage, which would have included floating turbines, were on the long list but not today's shortlist.
The biggest barrage proposal, an "outer barrage" which would have stretched from Minehead to Aberthaw, has not been included in the shortlist.
Neither has a barrage which would have been similar in size to the Cardiff-Weston scheme but would have landed at Hinkley rather than Brean Down on the English side.
And a 0.6-mile (1km) wide barrage on the Cardiff-Weston line, which would have had a wave farm and four marinas, has also been left off.
All 10 projects from the long list, and the proposed shortlist, will now be subject to a three-month consultation, after which the Government will publish a final shortlist.
Those five projects will be considered in more depth, with a view to making a final decision on how best to harness the energy of the Severn estuary in 2010.
The two-year multimillion-pound feasibility study by the Government aims to assess the costs, benefits and impacts of a tidal scheme in the Severn and identify a single preferred project from the options that have been proposed.
Welsh Assembly Government Environment Minister Jane Davidson said: "Harnessing the power of the Severn estuary ties could make a significant contribution towards achieving the UK targets for renewable energy and reducing carbon emissions, but we must ensure that environmental issues are taken fully into account.
"The shortlisted schemes are based on relatively well understood hydroelectric technologies, with a mix of existing and new engineering structures.
"It is proposed that the economic, social and environmental impacts of these be studied further in the second phase of the Government study."
The barrage proposals in particular have attracted controversy, with conservation groups concerned over the level of environmental damage they could do.
A study for the RSPB suggested the 12-mile "reef" could be cheaper and less damaging to wildlife than a barrage.
While a barrage would effectively dam the estuary, flooding huge areas of tidal habitat upstream of the construction, the reef would not hold back the full height of the tide and therefor have less impact on those sites.
Martin Harper, head of sustainable development at the RSPB, said: "It is hugely disappointing to see Government still pushing forward with the environmentally destructive option of a Cardiff-Weston barrage.
"We believe the focus should shift to innovative and potentially less- damaging alternatives like a tidal reef or tidal fence.
"The announcement of £500,000 to develop these schemes is very welcome, but it makes no sense to leave them off the shortlist.
"By excluding them, Government is excluding what could be the most environmentally benign options from its assessment of environmental impacts."
Natural England, the Government's conservation agency, said it was right to consider harnessing the power of the Severn estuary.
But the project should not go ahead without a detailed consideration of its environmental impacts and and a wider assessment of whether there were better ways to meet the drive towards renewable energy.
Helen Phillips, Natural England's chief executive, said: "We cannot sacrifice an environment as sensitive as the Severn estuary without resolving, once and for all, whether there are better alternatives.
"We need to look at renewable energy and energy conservation in the round and satisfy ourselves that tidal power in this area - with all the environmental consequences that go with it - really is the best route to take."
The Wildlife Trusts expressed concern that schemes where shortlisted at an early stage in the feasibility study, warning that it favoured proven technology - barrages - which would be highly damaging environmentally.
The Trusts said a barrage would destroy habitat on which species of bird such as shelduck, dunlin, redshank, teal, European white fronted geese and pintail depend.
Migrating fish species, such as salmon, trout and eel, would also be at risk.
The Trusts acknowledged the Government's commitment to funding for innovative technologies but said less environmentally damaging schemes such as the reefs should have made it on to the shortlist.
Mr Miliband and Ms Davidson presented the shortlist this morning at a stakeholders conference in the hands-on science museum in Bristol.
They were joined by minister of state for the DECC Mike O'Brien MP.
Addressing concerns that a number of the schemes would have a detrimental impact on the bio-diversity of the Severn estuary, Mr Miliband told a press conference: "The impact of catastrophic climate change, or dangerous climate change on biodiversity is extremely significant.
"If you had water levels rising by a metre for example, that would have a very bad, very negative effect on the Severn estuary.
"There are issues, of course, around the local environmental effects of the kind of technologies that are on the table.
"But I think you also have to take into account the wider climate effects and their potential impact on biodiversity. That's part of the process we are embarking upon.
"Of course climate change is overwhelming and important, but part of the next stage of the process is specifically looking at the biodiversity impact and the compensatory measure that might be necessary."
While coastal tidal lagoons were included on today's shortlist alongside the more conventional barrage proposals, Friends of the Earth Cymru criticised the Government's failure to pursue offshore lagoon proposals.
Friends of the Earth Cymru director Gordon James said: "Offshore tidal lagoons offer the best option for harnessing the huge renewable energy potential of the Severn estuary - their exclusion from the Government's shortlist is utterly incomprehensible and raises serious concerns about the consultation process.
"The development of tidal lagoons would have delivered huge quantities of green power more cheaply and quickly than a barrage, and with less impact on the environment.
"Ministers must abandon their fixation with the Severn barrage and invest in more effective and less damaging alternatives instead."Reuse content