By Mark Kinver
Science and nature reporter, BBC News
An influential UK government advisory body has endorsed proposals for a tidal barrage across the Severn estuary.
The barrage would stretch from south Wales to the Somerset coast
The Sustainable Development Commission (SDC) says the project should meet "tough" environmental conditions and remain in public ownership.
Last week the government announced an inquiry into the Severn scheme, which is opposed by a number of green groups.
The SDC, chaired by Jonathon Porritt, says the UK could get at least 10% of its electricity from tidal power.
Its report, Turning the Tide, was launched at parallel events in London and Cardiff.
"We are excited about the contributions a Severn Barrage could make to a more sustainable future, but not at any cost," Mr Porritt said.
"The enormous potential... to help reduce our carbon emissions and improve energy security needs to be balanced against the impact on the estuary's unique habitat, as well as its communities and businesses."
SEVERN BARRAGE STATS
Length: 16.1km (10 miles)
Generating capacity: 8.64 gigawatts (GW)
Annual average output: 17 terrawatt hours (TWh)
Percentage of UK electricity: 4.4%
Estimated cost: £15bn
The SDC outlined a number of conditions that would have to be met in order for the massive project to win the commission's support.
Firstly, the barrage had to be "publicly led as a project and publicly owned as an asset" to ensure that the government was responsible for the long-term sustainability of the project.
The report said that the lower rates of interest available to a government-led project would provide consumers with competitively priced electricity.
It also called for a large-scale "compensatory habitat" package to replace breeding and feeding sites for migratory birds around the estuary that would be lost if the barrage got the go-ahead.
The commission also said that it would only support the 16km-long structure if it was constructed within the parameters of existing European environmental legislation, such as the EU Habitats Directive.
Welsh Secretary Peter Hain said he welcomed the SDC's report, adding that the scheme was about "thinking big and acting bold".
"If we're serious about the fight against climate change, and if we're serious about clean, green energy, then this is the best example of it," he added.
"Climate change is the greatest challenge facing this country and the global community.
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
"If Britain is to play a leading role in meeting this challenge, projects such as the barrage cannot remain ambition forever."
He signalled that a multi-million pound feasibility study would go ahead, citing climate concerns as the main driver.
But environmental groups such as the Royal Society for the Protection of Birds (RSPB), Friends of the Earth (FoE) and WWF are not convinced.
FoE Cymru energy spokesman Neil Crumpton said: "The £14bn Severn barrage would be a hugely expensive, environmentally damaging and legally questionable mega-project."
These groups believe the scheme could devastate wildlife in the estuary, and suggest that tidal lagoons would be a more efficient and less damaging alternative.
Tidal lagoons are artificially created offshore pools. Water would flow through turbines in and out of the lagoons as the tide rose and fell.
Bernie Bulkin, the SDC's energy commissioner, said that the report had considered the role of lagoons, but concluded the concept remained unproven.
"We are not persuaded that this is the way to go," he told reporters at the report's London launch.
"There are places where a lagoon might make sense; where small lagoons might be a way of capturing renewable power.
"Our advice to government is that it is in the public interest to see if we can get one or more demonstration projects going, but we are not persuaded that this technology is an alternative to the Severn barrage."
Dr Bulkin said that report also highlighted that tidal power could be exploited to generate at least 10% of the UK's electricity needs.
He added that the UK had a "very good position" on tidal stream technologies, which uses the energy of the flow of tides to power turbines.
"Many devices have been developed in the UK to exploit energy from tidal streams. We have an excellent resource and we are probably the world's leaders today in tidal stream technology."
He suggested that the sector could provide a "significant source of export revenue" as more countries looked to utilise tidal power.
"The message in the report to the government is to stay on the course on this; it is an important resource and an important technology."