For more than 150 years, engineers have toyed with the idea of constructing a dam across the width of the Bristol Channel. But the dream of converting the estuary's massive tides into electricity by building the Severn Barrage has remained on the drawing board, scuppered by the cost and scale of the project – and the controversy that it has always managed to ignite.

The scheme took a large step towards reality yesterday, however, when John Hutton, the Secretary of State for Business, announced a multimillion- pound feasibility study into the construction of a barrage linking Somerset to South Wales, championing it as a "truly visionary" project which could put Britain firmly on the road towards cleaner, renewable energy.

The Government's glowing endorsement has revived the thorny dilemma long agonised over by green and conservation groups: how best to balance the competing advantages and disadvantages of damming the estuary. Is it worth sacrificing some of the mud-flats along the river, home to thousands of wading birds, in order to generate vast amounts environmentally-friendly energy?

Mr Hutton made clear where his loyalties lay, lavishing praise on the proposals and calling for a "step change" in the use of energy generated by the force of the tides, rivers and the wind. The Government shows every sign of wanting to approve the scheme as it alone could provide 5 per cent of Britain's energy through renewable power by the year 2020.

The plans have been drawn up by a consortium of major engineering companies and could take eight years to build, with 2017 the earliest realistic completion date. They envisage building a 10-mile structure from Brean Down, near Weston-super-Mare, to Lavernock Point, south of Cardiff. It would be one of the world's most ambitious engineering projects, building a series of sluices and turbines, along with shipping locks that would allow access to the ports of Cardiff, Newport, Bristol and Gloucester behind the barrage.

A road or rail link would open along the top of the structure, boosting the links between Devon and Somerset with the industrial centres of south Wales. If the technical problems of the £15bn scheme could be overcome, the benefits could be enormous, producing an astonishing annual output equivalent to the power generated by three nuclear power stations or 18 million tons of coal.

For a government committed to a sharp increase in renewable energy over the next decade, the attractions are obvious. It would also have the benefit of protecting the area against the increasing risk of flooding caused by global warming. Speaking yesterday to the Labour Party conference in Bournemouth, Mr Hutton said: "The government that Gordon Brown leads will not be among those which say they want to tackle global warming by moving to low carbon energy sources but then oppose every opportunity to do so." But the project has also alarmed conservationists, who warn that the mudflats and salt marshes that emerge at low tide provide food for 65,000 birds every winter.

The estuary has been designated a special conservation area, with legal protection, in recognition of its status as one of Britain's most important breeding areas for wading birds. The Wildfowl and Wetlands Trust, which runs a world-renowned haven for waterbirds at Slimbridge, Gloucestershire, said it recognised the potential benefits of harnessing the local tides.

But it warned: "The construction of a huge dam across the estuary could have a massive environmental impact on this delicate ecosystem and all of the wildlife that depends upon it." Mark Avery, conservation director of the Royal Society for the Protection of Birds (RSPB),said: "A barrage would do enormous damage and its layers of legal protection are there for good reason."

Both the RSPB and the Friends of the Earth (FoE) are supporting an alternative idea of tidal lagoons in the estuary, which would fill and drain through turbines. Tony Juniper, director of FoE, warned of the ecological damage that would be created by the barrage which he described as a "distraction, not a solution". Other opponents have warned that the construction of a barrage could create a build-up of sewage and would disrupt the famous natural phenomenon of the Severn bore, a wall of water which floods up the river and can often be ridden by surfers.

Mr Hutton promised that the feasibility study would examine the social and economic aspects of the scheme as well as its effects on the environment. It will also consider other sites in the UK where the power of the tides could be harnessed, including the Mersey estuary. But the benefits would be most pronounced in the Bristol Channel, where the estuary's funnel shape produces a tidal range of more than 14 metres, the second highest in the world after the Bay of Fundy on the east coast of Canada.

(By Nigel Morris, The Independent, 26/09/2007)