Launching its report on tidal energy, the Sustainable Development Commission (SDC) insisted that plans for a barrage across the Severn must comply with the EU habitats and birds directives, which protect the estuary.
Compensatory habitats would have to be created on an unprecedented scale to achieve this. But the commission is convinced that there is sufficient space to make this possible, although its report does not recommend any specific locations.
The study was due in the summer, but was delayed by consideration of legalities. SDC chairman Jonathon Porritt told Planning: "Tests for habitats under the directive are clear. They can be created in several places."
However, the Environment Agency claims that a Severn barrage would not be feasible within the directives' legal framework and thousands of hectares of inter-tidal habitat would be required.
"We have struggled to find a few hundred hectares. We cannot see where such an extent could be found," said Environment Agency sustainable development policy manager Richard Howell.
If proposals for the 16km barrage between Cardiff and Weston-super-Mare go ahead, 145ha out of 200ha of habitat would be lost.
Turning the Tide is available at PlanningResource.co.uk/doc.