The barrage would stretch from south Wales to the Somerset coast
Friends of the Earth (FoE) has reacted angrily to plans for large lagoons in the Severn Estuary being dropped from a shortlist of renewable energy schemes.
The environmental charity said it was "utterly incomprehensible" only smaller lagoon schemes were included.
Proposals for a 10-mile barrage from Lavernock Point, Vale of Glamorgan, to Somerset, made the preferred shortlist.
FoE Cymru director Gordon James said: "Ministers must abandon their fixation with the Severn barrage."
Two other barrage schemes and two lagoons, which section off the estuary but do not dam it, are also to be considered under the shortlist published on Monday.
They were drawn from a long list of 10 projects published last summer, which also included the proposals for larger lagoon schemes back by FoE.
A three-month public consultation on the feasibility of all 10 projects for harnessing tidal power in the Severn Estuary is underway, before a final shortlist is confirmed.
The preferred project is due to be selected in 2010.
FoE said the absence of the larger lagoon proposals raised "serious concerns" about the consultation process, which they claimed seemed to be little more than a cosmetic exercise.
"Today's announcement appears to confirm our suspicions," said Mr James.
"Offshore tidal lagoons offer the best option for harnessing the huge renewable energy potential of the Severn estuary - their exclusion from the government's shortlist is utterly incomprehensible and raises serious concerns about the consultation process.
"The development of tidal lagoons would have delivered huge quantities of green power more cheaply and quickly than a barrage, and with less impact on the environment."
A longlist published last year included plans for larger lagoons in the Severn Estuary
The Environment Agency said it would submit its response to the feasibility study within the consultation period.
Agency chairman Lord Chris Smith said: "The rivers Severn, Wye and Usk also include important ecological sites and protected species, and are some of the most important fishing rivers in Britain.
"The study must identify schemes that are environmentally-sensitive, but also help us meet renewable energy targets."
Natural England, the UK Government's conservation agency, said it was right to consider harnessing the power of the Severn estuary.
But the project should not go ahead without a detailed consideration of its environmental impacts.
Helen Phillips, Natural England's chief executive, said: "We cannot sacrifice an environment as sensitive as the Severn estuary without resolving, once and for all, whether there are better alternatives.
"We need to look at renewable energy and energy conservation in the round and satisfy ourselves that tidal power in this area - with all the environmental consequences that go with it - really is the best route to take."
The Wildlife Trusts expressed concern that schemes where shortlisted at an early stage in the feasibility study, warning that it favoured proven technology - barrages - which would be highly damaging environmentally.
Welsh Conservative environment spokesman Darren Millar AM said he was concerned plans for a tidal reef and a tidal fence had not been included in the preferred shortlist.
He said: "These are credible alternatives which could provide as much energy as a barrage but with less of an environmental impact.
"The UK and assembly governments need to keep an open mind on all possible options for harnessing the tidal energy of the Severn estuary."
Prof Ian Fells, fellow of the Royal Academy of Engineering, said the Cardiff-Weston barrage "will use proven technology, has by far the biggest potential for clean energy production, and the civil engineering companies are ready to start construction. So let's get on with it".
But Prof David Elliott, co-director of the Energy and Environment Research Unit at the Open University, said a single barrage was "pretty hopeless" in energy provision terms as it would only produce power in two bursts during a 24-hour period.
The tidal cycle powering the barrage might not coincide with demand for electricity.
Instead a series of small turbines around the coast, which would fire off at different times as tides vary around the UK, would offer more continuous supply, he said.