LONDON (Reuters) - The British government has extended financial support for offshore wind farms by four years to 2014, helping plant thousands more turbines over coming years as the country races to meet testing 2020 renewable energy targets.

Finance minister Alistair Darling said in his pre-budget speech on Wednesday that offshore wind farms accredited under the Renewable Obligation Certificates (ROCs) scheme before March 2014 would qualify for two ROCs per megawatt hour (MWh) of electricity they generate.

Britain, already home to the world's biggest offshore wind farms, hopes to install 33 gigawatts of turbines in its waters by 2020 in its efforts to source about a third of its electricity from renewables.

High costs of offshore wind make the technology unattractive to developers, prompting the government to increase incentives from 1.5 to 2 ROCs for projects accredited by April 2010.

Uncertainty over support levels for projects after next year has held back billions of potential investments.

"Keeping the two ROCs funding for offshore wind will help the UK retain its world lead and kickstart billions of pounds of investment ahead of the next major phase of offshore developments," Maria McCaffery, chief executive of renewable energy group the BWEA said.

"There are up to 3,000 megawatts worth of projects which should now benefit from this new support."