Birds like the Redshank would be affected, says the RSPB
Proposals for a controversial £14bn Severn barrage are set for consideration by the UK Government.
Supporters say the scheme could supply 5% of the UK's electricity, but environmentalists fear its impact.
The BBC News website gathered views from both sides of the argument.
ARGUING AGAINST THE BARRAGE
Martin Spray, chief executive of the Wildfowl and Wetlands Trust
"I can quite understand the view that perhaps this needs to be taken back into consideration.
"But I'm quite concerned about the Severn barrage, there is no doubt it would make quite a considerable impact to a natural, internationally important ecosystem.
"The whole of the Severn Estuary is home to some 65,000 various species of water birds and wading birds. It's a site of scientific interest."
Dr Mark Avery, conservation director, Royal Society for the Protection of Birds
"The Severn barrage was a bad idea in 2003 and is an even worse proposal now, not least because there are better, less damaging alternatives.
"Clean energy technology is developing apace and there will soon be less intrusive ways of harnessing the tides with little environmental cost. Tidal stream systems are being tested in the Bristol Channel, Scotland and Northern Ireland and could generate far more energy and cause little environmental harm.
"The RSPB strongly supports the development of renewables but today's white paper lacks the drive to develop the best green energy projects quickly enough."
Gordon James, campaigner with Friends of the Earth Cymru
"We are opposed to the large Severn barrage because it would have a major negative environmental impact.
"Any proposal to build a barrage could face strong legal challenges on environmental grounds.
"It would take a long time to build, there would be high costs and it would generate a lot of carbon dioxide as it was being built.
"There are better alternatives available such as a tidal lagoons."
Councillor Anthony Ernest, representing Lavernock and Sully
"The only objectors I have heard in the media so far are those who either favour renewable energy, or are concerned about a few rabbits and seagulls, who live around the coastline.
"Not one of those interviewed has dared talk about human misery, loss of property and the wiping out of huge areas of rural countryside, which currently protect the total integration of towns and the city itself, around Cardiff."
ARGUING FOR THE BARRAGE
Stephen Williams, Bristol West MP, sponsored an early day motion calling on the Government to look again at the barrage proposal
"I grew up in south Wales and I remember the barrage being discussed. The idea has been around for a long time.
"It has got great potential for creating clean energy. It is now extremely pressing and I think we should leave no stone unturned.
"The price of doing nothing about changing our energy mix, how we generate our energy and not reducing our carbon footprint means several years down the track the habitat of the Severn could be affected anyway. We've got to do something.
"The last time this was looked at in detail was 1989. We want a serious study into its potential and to say whether this could be best realised by a barrage or lagoon."
Paul Flynn, MP for Newport West, also signed the early day motion
"Tidal power is British, will carry on virtually forever, is a free source of energy and is carbon neutral.
"It has been vastly undersold, little understood and would provide us with huge quantities of electricity and is a far better prospect than nuclear power.
"It is environmentally benign and something we can take complete control of.
"The cheapest power produced in the world is in La Rance (a tidal barrage) in Brittany and it has been operating more than 30 years."
Jonny Boston of the South West Rural Development Agency
"It's probably more likely than it's been for perhaps 20 years - with the threat of climate change on us, we need to look at new ways of tackling that and generating our energy.
"This is now being looked at but if it did go ahead, it would still be nearly 2020 probably before any power was generated - and what we're doing at the RDA is pioneering some projects in the region to try to bring other renewable technologies to fruition which can also help tackle climate change."