British Telecom has announced a three-year plan to get all of its energy needs from renewable sources - the biggest such project in the world.
BT will get its energy needs from renewable sources such as wind
The electricity used, worth hundreds of millions of pounds, could power a city the size of Nottingham.
Renewable sources of energy include wind, wave and solar power, rather than using fossil fuels like oil or coal.
The power will be used in all of BT's 6,500 phone exchanges, as well as its satellite relay stations and offices.
The plan comes after the government's call for 10% of the UK's energy to have come from non-polluting renewable sources by 2010, in an effort to halt global warming.
Burning fossil fuels such as oil and coal are blamed by some scientists as a major contributor to rising temperatures.
BT's contract will be the biggest in history by a British company, though other UK firms have already signed up to using environmentally-friendly energy.
The UK is legally bound, under the Kyoto Protocol on climate change, to reduce emissions of greenhouse gases by 5.2%, below 1990 levels, by 2012.
In his foreword to the 2003 energy white paper, Tony Blair stated: "We are showing leadership by putting the UK on a path to a 60% reduction in its carbon dioxide emissions by 2050."
Two new wind farms which will generate enough power for 118,000 homes were given the go-ahead in Scotland last week.
Deputy Enterprise Minister Allan Wilson has announced turbines will be built at Braes of Doune, near Dunblane, and Farr, near Tomatin in the Highlands.