MPs say developers "failed to address commercial and environmental concerns" in the barrage plan and the scheme's viability isn't proven.
Bristol Port attack barrage project, but unions champion the economic boost to South Wales.
The Prime Minister instructs ministers to explore the development of a hydroelectric barrage across the River Severn.
A plan for a barrage across the Severn Estuary has been dealt a blow.
An influential committee of MPs has rejected the idea, claiming the economic and environmental case is unproven.
The consortium behind the scheme says it will bring billions in private investment and thousands of jobs.
Hafren Power, the developers behind the Severn Barrage plan, says today's critical report from the Energy and Climate Change Committee is "unhelpful and frustrating" - but says it will press on with the project.
The report is unhelpful and frustrating – we all know we have a lot more work to do and we will do it.
The Government has already told us it is not against the barrage and we are determined to press ministers and officials to engage fully. We believe the environmental and economic issues can be solved with everyone working together.
– Tony Pryor, Chief Executive, Hafren Power
Unlike smaller schemes, only a barrage can harness the full power potential of the estuary and do it economically. It will also be much cheaper and last much longer than offshore wind farms which have high levels of public subsidy.
Britain needs more infrastructure projects, especially power generation. Our proposal delivers in spades – up to £25 billion in private investment, 20,000 construction jobs and a further 30,000 jobs supported, and the cheapest zero carbon electricity over its lifetime.
The plan is for an 18km (11-mile) barrage between Lavernock Point, near Penarth in the Vale of Glamorgan, and Brean, near Weston-super-Mare.
The developer, Hafren Power, says it plans to "harness one of the world's largest potential sources of renewable energy: the huge tidal range of the Severn estuary" - in "one of the largest privately funded engineering projects in the world."
It says the barrage will produce 16.5 terawatt hours per year (around five percent of the UK's electricity demand) for over 120 years.
It would be one of the biggest power stations in Europe, equivalent to 3,000 wind turbines or three to four nuclear reactors.
Hafren Power says the barrage would be clean, secure and sustainable.
Plans for a Severn Barrage are under scrutiny at Westminster today. Supporters say it'd create jobs and provide five percent of the UK's energy needs.
But enviromentalists warn it could be devastating for wildlife.
Owain Phillips reports.
The UK Energy Secretary Ed Davey says a 30 billion pound barrage across the Severn estuary could still be built if the right proposals were submitted.
The barrage would stretch from the Vale of Glamorgan to Somerset. Supporters believe it could provide 5% of UK electricity - but some MPs and environmental groups have opposed the plans.
Plans to build a barrage across the Severn Estuary could be back on the agenda after the UK Government confirmed it was open to working with affordable and environmentally friendly projects.
Harnessing the tidal power could provide 5% of the UK's energy needs.
Environmentalists have urged caution , amid fears over the scheme's potential to damage the natural habitat for wildlife in the area.
"We accept that the power of the Severn could well be used, but we'd like to see it done in a way that will minimise the impact on wildlife " says Katie-Jo Luxton, from RSPB Cymru.
Plans to build a Severn Barrage appear to be back on the agenda - after the UK Government said it was open to affordable and environmentally friendly projects.
The Prime Minister is believed to have discussed the project with the MP and barrage supporter Peter Hain.
Some are sceptical that this is real progress, as Nicola Hendy reports.