New Environment Agency boss Lord Smith has attacked proposals for the controversial Severn Barrage, claiming it will destroy fish stocks and wreck bird habitats.

The former Cabinet minister also used his first interview in his new position to warn that stretches of Britain's coastline will have to be abandoned and people evacuated from the most threatened areas.

His outspoken denunciation of the ambitious plans for a massive Severn Barrage will embarrass Business Secretary John Hutton, who has signalled his enthusiasm for the project.

As Chris Smith, Lord Smith was Culture, Media and Sport Secretary under Tony Blair, and he remains influential within Government circles.

Ten different schemes are being considered to harness the huge tidal power of the Severn Estuary.

They include five barrages, including the best-known idea of a £15-billion, 10-mile one from Brean Down, near Weston-super-Mare, to Lavernock Point, near Cardiff, and an even larger one from Minehead to Aberthaw.

There are also smaller and cheaper projects currently being examined in a feasibility study, including a tidal fence, lagoons and a "lake" scheme that would feature a wave farm and four marinas. A shortlist of preferred options to be taken forward for more extensive research will be announced in December, when the two-year feasibility study ends.

Lord Smith did back more modest schemes to use the Severn's unique tidal power to generate electricity, but said he was alarmed at the Government's support for a fixed barrier.

"Effectively you would be destroying the fish populations of everything up the river system from the barrier," he said.

"That is a major environmental downside."

His intervention follows former Agency chief executive Baroness Young, who said last December that a barrage across the Estuary's "irreplaceable habitat" would put protected sites across Europe at risk. She said the barrage set a precedent and trigger "open season" on protected sites, and was worried Mr Hutton was not neutral on the barrage, but "quite excited" about it.

Supporters say a barrage could generate five per cent of the UK's electricity and provide up to 40,000 jobs.

Ministers insist they have not come to a conclusion about the feasibility study.

Lord Smith also said Britain faced hard choices about which areas of the coast to defend, as some places were doomed and would have to be left to be reclaimed by the sea.

He said it would be impossible to protect every part of the coast, with stretches of north-east Norfolk and Suffolk most at risk.

"This is the most difficult issue we are going to face as an agency. We know the sea is eating away at the coast in quite a number of places, primarily – but not totally exclusively – on the east and south coasts. It's a particularly huge issue in East Anglia, but in quite a number of other areas as well."

His comments suggest it could become an issue for the World Heritage Jurassic Coastline off Dorset and Devon, which had a massive landslide in May.

Shadow Environment Secretary Peter Ainsworth said: "Chris Smith is right – even with modern technology, and even if we had all the money in the world, we cannot hold back the sea everywhere.

"I fully understand the intense anxiety in local communities affected by coastal erosion. This is made much worse by an intolerable uncertainty over Government policy. It is essential that ministers act swiftly to provide reassurance where it can be offered, and to engage in an honest debate with people whose properties may fall to the irresistible force of nature."