th Wales, could produce power for about £100 per megawatt hour (MWh).

Three tidal lagoons could be in operation around Britain by 2021 producing large-scale low carbon power at a cheaper price than offshore wind, according to their developer, Tidal Lagoon Power.

The company said a report commissioned from management consultant, Pöyry confirmed the lagoons, starting with one at Swansea Bay in south Wales, could produce power for about £100 per megawatt hour (MWh).

"This study clearly demonstrates that tidal lagoons can rapidly become one of the cheapest sources of electricity in the UK. The more water we impound, the more power we produce, the less support we require," said Mark Shorrock, chief executive of Tidal Lagoon Power.

The £100 per MWh cost compares with recent government calculations for a deep sea offshore wind farm in 2021 of about £131 per MWh. The cheapest of the projects assessed, Lagoon 3, has a cost of about £90 per MWh, broadly similar to onshore wind, large-scale solar and even gas-fired plants, Shorrock said.

This month, plans for the world's first tidal lagoon power plant in Swansea Bay were accepted for consideration by the government's Planning Inspectorate. Tidal Lagoon Power is hoping construction can start in spring 2015 with the first power generated in 2018.

Environmentalists, who have scuppered other tidal projects such as the Severn Barrage in the past, remain cautious about the impact of the scheme on wildlife and a former business partner, Tidal Electric, claims to be in dispute with the Swansea Bay developers over the design of the scheme.