Plans for a controversial £30bn Severn barrage tidal energy project stretching from Weston-super-Mare in Somerset to Cardiff are expected to be scrapped.
Secretary of State for Energy Chris Huhne is expected to make the announcement in Parliament on Monday.
Reports suggest the scheme is to be axed as it is not "financially viable" and that instead he will give the go-ahead to new nuclear power stations.
The Department of Energy and Climate Change could not confirm or deny it.
Supporters of the tidal project, which would link Lavernock Point near Cardiff, to Brean Down near Weston-Super-Mare, claimed it could generate 5% of Britain's electricity.
Dr Rob Kirby, an independent expert on the Severn Estuary, has worked on the project for the last 40 years.
Dr Kirby, who has worked with the Department for Energy and Climate Change on the Severn barrage, said: "The government's view is that it's too big a project to approach in financial terms.
"Raising the money in this financial climate would be too much of a challenge.
"The barrage has been killed off for the moment by environmental fundamentalism because environmentalists have always objected to the Severn barrage.
"It's quite unambiguous - the Cardiff to Weston (barrage) can only benefit the environment and those who say otherwise are not telling the truth."
Shadow Welsh Secretary Peter Hain said scrapping the barrage would be "equally disastrous" for the economy and the environment.
"Not only is Chris Huhne turning his back on the proposed barrage scheme that would have created hundreds of good quality green jobs for Welsh people, it appears that he decided to abandon in its entirety the idea of using the Severn estuary as a generator of electricity.
"The proposed barrage would have produced 5% of the UK's energy needs - equivalent to two nuclear power stations."
The proposals caused concern among environmental groups, including the RSPB and Friends of the Earth Cymru (FoE), concerned about the impact on wildlife in the estuary.
FoE said the proposals would have caused "irreversible damage" to wildlife, while harnessing the power of the Severn's tides with lagoons and reefs were "better options" which needed to be urgently looked at.
Chris Witts, a Liberal Democrat councillor for Gloucester City Council and a River Severn historian, said: "I'm delighted if the barrage is to be scrapped.
"We're messing with nature in too big a way. I hear stories from around the world that barrages have created problems and I wouldn't want to see problems created on the Severn.
"I'm not against getting energy from the Severn but not with a barrage on this scale."
The 10-mile (16km) barrage proposal - known as the Cardiff-Weston barrage - is one of five shortlisted schemes to harness renewable energy from the tides of the Severn Estuary, which has the second-largest tidal range in the world with 42ft (12.8m) tides.
The barrage would harness water power using a hydro-electric dam, but would be filled by the incoming tide rather than by water flowing downstream.