The telecoms giant will select a site to launch the trial in the spring.

The rooftop generator, built by UK-based Wind Dam, will supply most of the power to the telephone exchange during the test, which will take several months to assess.

BT is believed to be the first leading company investing in the rooftop turbines as part of its drive to increase the amount of renewable energy it uses.

Power demand

The move followed an announcement by the government earlier this month of a huge expansion in offshore wind farms.

Plans for the world's largest offshore wind farms moved a step closer when 15 new sites around the UK coastline were announced.

The wind farms are expected to produce electricity for more than one in six households by the end of the decade.

The sites will be built in three areas of shallow sea around the Thames Estuary, Greater Wash and the North West.

BT accounts for 1.8% of the non-domestic electricity generated in the UK and expects to use more power as its businesses, including broadband internet use, increase.

An independent report commissioned by BT earlier this year advised the company to invest in new renewable technologies.

"We hope to build a business case to install rooftop wind generators in other buildings," said a BT spokesman.