The Didcot site is the second most polluting power station in Britain - behind only Drax in Yorkshire.

Greenpeace campaigns director Blake Lee-Harwood said the station emits more than six million tonnes of carbon dioxide a year - more than the 29 lowest polluting countries put together.

The organisation claims that the station could halve emissions overnight if it switched from burning coal to gas.

Greenpeace says power stations like Didcot waste two-thirds of the energy they generate in the form of heat escaping from their cooling towers.

They say smaller generators should be built closer to where energy is used, with more heat captured.

The occupation comes in the week Sir Nicholas Stern released a study warning of a global catastrophe if carbon emissions are not slashed.

Ben Stewart, occupying the 200m-tall chimney, added: "We'll leave this power station when Tony Blair pledges to ditch these dinosaurs and start investing in cutting-edge decentralised energy."

Didcot A Power Station manager John Rainford said the plant was due to be phased out by 2015. He said nPower - which runs the station - was investing in alternative technologies.

A new bio-fuel plant, costing £3.5m, will allow the station to cut its carbon dioxoide emissions.

The plant, which has gained Environment Agency approval, will mix predominantly sawdust and palm oil products with coal.

Neil Sweeney, of nPower's green fuel division, said last month that though it is only a one per cent cut in carbon emissions "if everyone could reduce by one per cent, that would be a serious step forward."