Three estuaries have been earmarked by the government as Special Areas of Conservation (SACs) to protect vulnerable wildlife and habitats.
The Severn Estuary has the second highest tidal range in the world
Defra has written to the European Commission to seek SAC status for the Severn, the Dee and the Humber.
The Severn Estuary has been selected as one of the best areas in the UK for mudflats, sand flats and Atlantic salt meadows.
It is also important for migratory fish and a nursery for juvenile fish.
The estuary's classic funnel shape, unique in the UK, helps give it the third highest tidal range in the world at more than 12 metres.
The Dee Estuary supports extensive areas of salt marsh and has the fifth largest extent of mudflats and sand flats of any estuary in the UK.
It includes a dune system along the north-east coast of Wales which supports a rich variety of plants, including the rare petalwort.
The estuary is also important for a number of migratory fish species.
The Humber Estuary is the largest British coastal plain estuary on the North Sea coast, and drains one fifth of England.
The subtidal and intertidal habitats, fringing salt marsh and reedbeds provide a valuable resource for a large number of rare or threatened mammals, fish and plants.
The Sustainable Development Commission is due to report in the autumn on the potential for utilising tidal power to generate electricity both in the Severn and elsewhere.
SAC designation would not rule out tidal power development in the Severn Estuary.