U.K. fuels for cars and trucks must contain biofuels starting now, a move that may do more harm than good to the environment and drive food prices higher, charities including Oxfam and Greenpeace said.

Under the Renewable Transport Fuel Obligation, suppliers must ensure that 2.5 percent of fuel sold at U.K. pumps consists of biofuels, which are made from crops and grasses. The requirement will rise to 5 percent by 2010. The Department for Transport says the plan will cut carbon-dioxide emissions by 2.5 million metric tons a year.

Scientists in the U.K. and U.S. have found that the cultivation of biofuels can increase the output of CO2 and other gases blamed for global warming because of changes in land use. U.K. Prime Minister Gordon Brown wrote to leaders of the Group of Eight nations to say the government is concerned that biofuels are pushing up food costs around the world.