A leading marine charity has revealed its opposition to a proposed offshore barrage from the Westcountry to South Wales, saying it will leave households with hefty bills and cause environmental destruction.
The Marine Conservation Society has warned against the Severn estuary green energy project after it emerged that David Cameron has asked civil servants to examine new plans.
While the Government has previously rejected pumping taxpayers' money into the Bristol Channel scheme, which could cost up to £34 billion, a new proposal would be entirely funded by private funds. Investors in Kuwait and Qatar are anticipated to provide much of the cash.
Ministers would still need to give the consortium support in principle to pass planning and regulatory hurdles.
But the Marine Conservation Society says environmental concerns are "taking a back seat to what many will see as an impressive civil engineering project".
Mooted for decades, a barrage fitted with turbines would harness power locked within the Severn's massive tidal range, and represent the world's largest-ever green energy project.
On the Westcountry side, sites previously discussed include Bridgwater and Weston-super-Mare.
The charity fears "mutilated fish" will wash up along huge stretches of coastline, and that Somerset beaches including Weston and Brean Sands could end up being "strewn with dead fish".
Dr Robert Keirle, the society's pollution programme manager, also warned electricity produced will cost up to twice as much as energy produced by other technologies, which would hit household bills.
He said: "Fish and eel migratory routes will be severely inhibited. In fact they may be blocked completely; approximately 14,000 hectares of inter-tidal habitat of international importance lost upstream of the barrage, due to the high suspended sediment levels within the water. It is doubtful whether the lifetime of the barrage would be anywhere near as long as the projected 120 years; and the barrage could severely damage the viability of ports such as at Avonmouth."
He added: "In Strangford Lough in Northern Ireland locals believe the marine turbine has led to seals being decapitated and a similar scenario may be experienced in the Severn estuary."
Corlan Hafren, the consortium of engineering and construction companies behind the proposal, has entered talks with groups such as the RSPB, which has long questioned the environmental impact.
The estuary scheme is being championed by the former Labour minister Peter Hain and could generate 5 per cent of the UK's electricity and create 10,000 jobs.