The government of the United Kingdom today gave the formal go-ahead for two major offshore wind farms to be built in the Thames Estuary. And yet while it's a major step, it's not quite the last hurdle for the two massive projects.

The London Array and Thanet schemes together are to total 1.3GW of green electricity, enough to power a third of London's three million households when fully operational.

Politicians say they will make a significant contribution to the government's goal to deliver a five-fold increase in the U.K.'s renewable energy resource by 2020.

"Together, once built, they will mark a significant stride towards our renewables target. It is a significant step forward in providing a greener and clean source of power," said U.K. Secretary of State for Trade and Industry Alistair Darling.

"Britain is second only to Denmark in the offshore wind sector and projects such as the London Array, which will be the biggest in the world when completed, and Thanet underline the real progress that is being made."

"We expect this announcement will be the first of a number of large-scale offshore wind farms in the U.K. and will provide real impetus for the continued developments in the offshore renewable energy sector," said U.K. Environment Secretary David Miliband.

"By issuing the licences to build the world's largest offshore wind farms in the Thames Estuary we are re-enforcing the U.K.'s commitment to renewable energy and combating climate change and ocean acidification."

The London Array, to be built at an estimated cost of up to £1.5bn, will consist of 341 turbines - each capable of generating between 3-7MW - five offshore sub-stations and four meteorological masts. They will rise from the sea 20km off the Kent and Essex coasts (east side of the island) and will occupy an area 232km square in size stretching between Margate and Clacton.

The London Array is to be built by a consortium headed by energy giant Shell.

The £500 million Thanet wind farm is to be located approximately 11.3km from North Foreland on the Kent coast, just south of the London Array, and its 100 turbines are to occupy an area of around 35km square. The project, led by developer Warwick Energy, is being fast-tracked for delivery in 2008 and expects to provide electricity for around 240,000 homes.

Environmental group Friends of the Earth endorsed the projects, but told the government it "must go further" in cutting carbon emissions.

The Royal Society for the Protection of Birds (RSPB), which has been fighting the construction another windfarm proposed in Scotland, also gave its backing to the projects after plans were modified to protect an endangered bird, the red-throated diver.

The project isn't quite fully ready to proceed, however.

Local concerns over increased traffic and noise meant that a local council blocked the consortium's planning application for the onshore substation in July this year. An appeal was lodged, but development as planned is still dependent on the outcome of a hearing in March.

And Shell has cautioned that consent for the onshore substations necessary to connect the London Array into the national grid technically remains outstanding and is still subject to a public inquiry.

However, the U.K. Department of Trade and Industry (DTI) said it was confident that the windfarms would be built by 2011.

The two projects are expected to bring significant economic opportunities to the local communities and to U.K. businesses, the government said.