The combined cost of building the two wind farms will be about £2bn

The government said the schemes would produce enough renewable electricity to power about one million households.

The larger London Array project covers 90 sq miles (232 sq km) between Margate in Kent and Clacton, Essex.

The second wind farm, called the Thanet scheme, will cover 13.5 sq miles (35 sq km) off the north Kent coast.

The £1.5bn London Array scheme will have 341 turbines rising from the sea about 12 miles (20km) off the Kent and Essex coasts, as well as five offshore substations and four meteorological masts.

The consortium behind it is made up of Shell WindEnergy Ltd, E.ON UK Renewables and Core Ltd.

Climate change

The smaller £450m Thanet project will be located seven miles (11km) out from North Foreland, Kent, and will have 100 turbines.

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The government said both schemes would make "a significant contribution to the aim of a five-fold increase in the UK's renewable energy resource by 2020".

Friends of the Earth said about 1% of electricity used in the UK would come from the London Array turbines.

Spokesman Martin Williams said: "It really is a big wind farm, but when you look at the scale of the challenge of climate change, we're going to need to do an awful lot more than just this one offshore wind farm."

And the RSPB backed the London Array scheme after plans were amended to prevent harm to red-throated divers - a bird rarely seen in UK waters.

The charity said 7,000 of the birds were found off north east Kent between 2002 and 2005, a larger population than thought, and so developers reduced the number of first phase turbines.

Dr Mark Avery, conservation director at the RSPB, said: "We are very pleased that this wind farm is to be built. Renewable energy generation is crucial to tackling climate change."

New jobs

London Array Ltd also wants to build an onshore electricity substation at Cleve Hill, Graveney, but planning permission was refused by Swale Borough Council.

The consortium is appealing, saying it has changed its plans following residents' concerns over construction traffic, and a planning inspector is due to make a ruling in early March.

The Thanet scheme will provide major economic benefits for the town of Ramsgate.

Thanet District Council said 800 new jobs were expected during the three-year construction period, with another 100 when Ramsgate becomes the maintenance base for the wind farm.