Speaking in the House of Lords, Baroness Young said the barrage – which is supported by the Wales Office and the Welsh Assembly – would create as many environmental problems as it would solve.

 “Its irreplaceability is due to the extent of its tidal range and this proposal would reduce considerably the tidal range of a very large part of the estuary. It is one of the more important estuarine systems in Europe for things like its salmon rivers and inter-tidal rivers. It is truly a globally unique river,” she said.

She added: “If we were to over-ride the protection of one of our most highly protected sites in this country, I believe it would become open season.”

The £15 billion plan to create a barrage running from the Vale of Glamorgan Somerset would, say its supporters,  generate up to 5 per cent of the UK’s energy needs. The Government is carrying out a feasibility study into the idea and a update on this is due early in the new year.