The report, commissioned by the RSPB, concludes that a longer dam further downstream would preserve most of the estuary's saltmarshes and mudflats, where at least 68,000 birds feed in winter. The government is considering ten tidal proposals and is due to decide next month whether to pursue a barrage.
A Department for Energy and Climate Change spokeswoman admitted that there is a chance that ministers may decide to go no further. "We have always said we would have a review of where the project is at. We will take stock to see whether it is financially and environmentally feasible," she said.
Atkins found that a tidal reef would cost £2 billion less than the controversial Cardiff-Weston barrage proposal. If ministers decide to continue the project they will announce a shortlist of schemes in the new year.