The tide moves a huge amount of water twice each day, and harnessing it could provide a great deal of energy - around 20% of Britain's needs.

Only around 20 sites in the world have been identified as possible tidal power stations.

There are eight main sites around Britain where tidal power stations could usefully be built, including the Severn, Dee, Solway and Humber estuaries.

There are four types of tidal power - Tidal Stream Energy devices, Tidal Fences, Tidal Lagoons and Tidal Barrages.

The impacts on wildlife and sediment regime, the resources needed, the uncertainties and construction times increase with type in this order and environmentalists argue for tidal stream turbines for minimal impact to the environment.

Sufficiently fast flowing water is found in rivers, estuaries and tidal flows and its energy can be converted by the tidal stream turbine into electricity.