Biofuels - petrol and diesel extracted from plants - are presented as an environmentally friendly alternative to fossil fuels because the crops absorb carbon dioxide from the atmosphere as they grow.

The study warns that forests must not be cleared to make way for biofuel crops. Clearing forests produces an immediate release of carbon gases into the atmosphere, accompanied by a loss of habitats, wildlife and livelihoods, the researchers said.

Britain is committed to substituting 10% of its transport fuel with biofuels under Europewide plans to slash carbon emissions by 2020.

"Biofuel policy is rushing ahead without understanding the implications," said Renton Righelato of the World Land Trust, a conservation charity. "It is a mistake in climate change terms to use biofuels."

Dr Righelato's study, with Dominick Spracklen from the University of Leeds, is the first to calculate the impact of biofuel carbon emissions across the whole cycle of planting, extraction and conversion into fuel. They report in the journal Science that between two and nine times more carbon emissions are avoided by trapping carbon in trees and forest soil than by replacing fossil fuels with biofuels.

Around 40% of Europe's agricultural land would be needed to grow biofuel crops to meet the 10% fossil fuel substitution target. That demand on arable land cannot be met in the EU or the US, say the scientists, so is likely to shift the burden on land in developing countries.

The National Farmers Union said 20% of Britain's agricultural land could be used to grow biofuels by 2010. However, the researchers say reforesting the land would be a better way to reduce emissions.

Biofuels look good in climate change terms from a Western perspective, said Dr Spracklen, but globally they actually lead to higher carbon emissions. "Brazil, Paraguay, Indonesia among others have huge deforestation programmes to supply the world biofuel market", he said.