Two estuaries in north and south Wales have been designated Special Areas of Conservation (SACs) to protect vulnerable wildlife and habitats.
The new status for the Severn and Dee estuaries comes after a request from the Department for Environment, Food and Rural Affairs (Defra).
Both estuaries are important areas for migratory fish.
Defra said the European Commission had "adopted" the estuaries a year ago, but the decision had been made "formal".
SACs are designated because of possible threats to important habitats and wildlife.
The Severn was 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 12m (39ft).
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.
Confirmation that the Dee Estuary was to be formally designated an SAC came in the Commons in response to a question from Ben Chapman, the Labour MP for Wirral South.