By Gareth Jones
Business Editor, BBC Wales
A barrage across the Severn Estuary which supporters say could produce huge amounts of clean energy will probably never be built, it has been claimed.
Plans for a barrage across the Severn have provoked controversy
River engineering expert Fred Pearce told BBC Wales the project will cause severe environmental damage if built.
But another author, scientist James Lovelock, said clean energy projects were needed to avoid climate change.
The 10 mile-long tidal barrage could be constructed between Lavernock Point, near Cardiff, to Brean Down, Somerset.
Mr Pearce was at the Hay Festival to talk about his book When the Rivers Run Dry, which examines the often negative effect dam-building, irrigation and other human activities have had on water-courses around the world.
In a recent speech, First Minister Rhodri Morgan told a conference that the barrage could be a Welsh Three Gorges, referring to the world's biggest hydro-electric project nearing completion on the Yangtze River in China.
The barrage would stretch 10 miles across the Bristol Channel
Mr Pearce, who also writes for New Scientist magazine, told BBC Wales that the comparison was unfortunate.
"It'll probably cost as much and do as much environmental damage," he said.
The Welsh assembly government wants Whitehall to carry out a feasibility study into the barrage and says some big companies are interested in investing in such a scheme.
It says by using the massive movement of tides in the estuary, the project could avoid the need to build several nuclear power stations.
The assembly government is opposed to more nuclear plants in Wales.
There has been support for the idea of a barrage, however, from another author appearing at the festival.
In his book The Revenge of Gaia, scientist James Lovelock said there was an urgent need for major clean energy projects like the barrage if the world is to avoid catastrophic climate change.
He said that money must be made available "for practical renewable energy schemes such as the Severn Estuary tidal barrage."
He added: "This might provide a steady five to 10% of the energy needs of our nation when we stop the present wasteful misuse."
Mr Lovelock has been described as "one of the great thinkers of our time."
His1979 book, Gaia, made him a hero among environmentalists for the way it described the earth as something resembling a self-regulating organism that was under threat from human activity.
However, his recent calls for a revival of nuclear power in the UK have lost him friends in the environmental movement.
His calls for a Severn barrage could prove equally controversial.