The London Array and Thanet wind farms together total 1.3 gigawatts of green electricity, enough to power a third of London's three million households when fully operational, the government said.
Environment Secretary David Miliband said more offshore wind farms are in Britain's future. 'We expect this announcement will be the first of a number of large-scale offshore wind farms in the UK and will provide real impetus for the continued developments in the offshore renewable energy sector that will benefit generations to come,' he said.
Two government departments gave the projects permission to proceed - the Department of Trade and Industry, DTI, and the Department for Environment, Food and Rural Affairs, DEFRA.
'By issuing the licences to build the world's largest offshore wind farms in the Thames Estuary we are re-enforcing the UK's commitment to renewable energy and combating climate change and ocean acidification,' Miliband said.
The London Array will consist of 341 turbines - each capable of generating between three and seven megawatts of power. There will also be five offshore sub-stations and four meteorological masts.
Over a four year period, they will rise from the sea 20 kilometers, or 12 miles, off the Kent and Essex coasts and at completion will occupy an area 232 square kilometers in size stretching between Margate and Clacton.
Development is planned in two phases to allow bird impacts from the first phase to be assessed before confirmation of the second phase is given.
The consent for an onshore substation, necessary to connect London Array into the national grid, remains outstanding and will now be subject to a public inquiry. The consortium behind London Array is appealing against a Kent council's refusal for an onshore substation.
The government also gave the go-ahead to the Thanet windfarm, 11.3 km or seven miles from North Foreland on the Kent coast. The Isle of Thanet is an area at the most easterly point of Kent. The £500 million Thanet wind farm will consist of up to 100 turbines.
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.
The two projects will contribute to the government's stated ambition - set out in the Energy Review - to deliver a five-fold increase in the UK's renewable energy resource by 2020.
Secretary of State for Trade and Industry Alistair Darling said, '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.'
'Achieving rapid growth in offshore renewables is essential if we are to reduce carbon emissions and improve the security of our energy supplies,' said Darling.
Friends of the Earth welcomed the decision to grant offshore planning permission to the windfarms in the Thames Estuary.
Friends of the Earth's climate campaigner, Martyn Williams, said, 'There are lots of clean, safe solutions to the climate change challenge, but we must start investing in them now.'
'UK carbon dioxide emissions have risen under Labour,' Williams said. 'The government must show that it is prepared to take action that will lead to substantial cuts. Schemes like London Array can play a significant part, but we must go further. Ministers should start by ensuring that the recently-announced new climate change law will require successive governments to make annual cuts in UK emissions.'
The consortium behind London Array includes CORE Limited, E.ON UK Renewables and Shell WindEnergy Limited.
Welcoming the consent decision, James Smith, chair of Shell UK said on behalf of the developer's consortium, 'The London Array offshore wind farm will make a crucial contribution to the UK's renewable energy targets. We're delighted to have received the DTI's consents today.'
'Moreover, the government's decision to reform the planning system for major energy infrastructure projects will help ensure a balance is struck between the national interest and local concerns,' said Smith.
Environment Secretary Miliband said, 'Working with the developers and those engaged in the broader environmental debate throughout the challenging application process ensured that all the environmental issues were addressed, as well as the impacts to the marine environment.'
The ecology in the inter-tidal areas close to shore differs from that found in the deeper waters on site, so a further assessment of the ecology in this area was undertaken. The result of this work has led to re-routing of the proposed cables to avoid sensitive areas.
Welcoming the permissions, the British Wind Energy Association’s Chief Executive, Maria McCaffery said, 'The significance of London Array is far greater than the project itself, although this will bring many notable benefits to the UK in terms of clean, carbon free generation.'
'Far more important,' said McCaffery, 'is the clear signal from the UK to the rest of the world that this country is open for business for offshore wind and we look forward to more consents in the near future.'