Ministers are now studying the feasibility report into plans to harness the tidal energy of the Severn Estuary – and are signalling they will not back a £21 billion barrage.

The coalition Government will formally respond when Parliament returns after the summer recess to the document, which follows a two-year consultation.

But Tory Energy Minister Charles Hendry has warned the costs of the barrage are rising at a time when the nation's finances are at their tightest for decades.

He told an energy conference in Norway: "We have been studying responses to consultations and we are looking at that and we are due to publish a report in the near future.

"We are also looking at the likely costs and these have been escalating over time."

His comments echo those of his Liberal Democrat boss, Energy and Climate Change Secretary Chris Huhne two months ago, as reported by the Western Daily Press.

He said then: "In principle, I am very much in favour of everything we can do to make this country more energy independent and to provide security of supply, but there have to be considerations of costs as well.

"That is one of the things the feasibility study is looking at." The previous Labour government was keen on a barrage, but significantly the coalition has not issued any statements about tidal energy schemes, and there was no mention in its policy agreement.

And the Lib Dems voted at their party conference last year to reject the most controversial proposal, the £21 billion ten-mile crossing from Weston-super-Mare to Cardiff.

It could provide five per cent of the UK's electricity, but opponents claim it would damage a unique and fragile ecosystem.

Other schemes currently being considered include a smaller £3.2 billion 'shoots' barrage, and lagoons, which could harness the estuary tides of up to 14 metres.

The £21 billion includes construction and 'compensatory habitat costs' but not connecting to the national grid, and possible other environmental implications, such as changes to flood defences.