The government pledged its offices would go "carbon neutral" by 2012 -- meaning they will prevent as much carbon emissions as they produce.

They will do this by reducing emissions of greenhouse gases believed to cause global warming as much as they can.

Remaining emissions will be "offset" by investing in renewable energy or energy-efficiency projects.

The initiative will save about 800,000 tonnes of carbon -- equal to taking 750,000 cars off the road, the government said.

The government set a second goal of reducing carbon emissions from its offices by 30 percent by 2020.

Environmentally-friendly projects already adopted by one government department included a solar-controlled intelligent light system, that turns off lights when they are not needed, while another obtained a large part of its electricity from renewable sources such as wind power, a spokesman said.

"We are taking another important step today when it comes to 'greening' government," Blair said in a statement.

"I want the public sector to take a lead on doing things sustainably -- through the way we run central government and through the way we buy goods and services," he said.

Environmental group Friends of the Earth called Blair's pledge "an encouraging first step in the right direction".

"But he must match efforts to 'green' government departments with action to address rising UK emissions. That means introducing a climate law that will commit the government to cutting the UK's carbon dioxide emissions by three percent year-on-year," Friends of the Earth Director Tony Juniper said.

Blair has made tackling global warming a priority for his government and had set a domestic target for cutting emissions that was much more ambitious than Britain's commitment under the international Kyoto Protocol.

The government admitted in March it would miss its target, drawing scorn from environmentalists.