The government has unveiled plans to build enough off-shore wind farms to power every home in Britain.

Business secretary John Hutton announced the major expansion in renewable energy during a speech in Berlin on Monday.

Up to 7,000 turbines could be installed around the UK's coastline - more than two per mile.

Currently wind is the source of only one gigawatt of power in the UK, and the expansion would give Britain an off-shore wind industry twice the size of any other nation in the world.

Hutton said the draft plan could allow firms to develop up to 25 gigawatts of offshore wind by 2020, on top of the eight gigawatts already planned.

He said the expansion would come after an evironmental assessment.

"But if we could manage to achieve this, by 2020 enough electricity could be generated off our shores to power the equivalent of all of the UK's homes," the minister added.

"This could be a major contribution towards meeting the EU's target of 20 per cent of energy from renewable sources by 2020.

"The challenge for government and for industry is to turn this potential - for our energy and economy - into a cost-effective reality. This will be a major challenge."