It used to be a matter of good intentions gone awry. Now it is plain fraud. The governments using biofuel to tackle global warming know that it causes more harm than good. But they plough on regardless. In theory, fuels made from plants can reduce the amount of carbon dioxide emitted by cars and trucks. Plants absorb carbon as they grow - it is released again when the fuel is burned. By encouraging oil companies to switch from fossil plants to living ones, governments on both sides of the Atlantic claim to be "decarbonising" our transport networks.

In the budget last week, Gordon Brown announced that he would extend the tax rebate for biofuels until 2010. From next year all suppliers in the U.K. will have to ensure that 2.5% of the fuel they sell is made from plants - if not, they must pay a penalty of 15 pence a liter. The obligation rises to 5% in 2010. By 2050, the government hopes that 33% of our fuel will come from crops. Last month George Bush announced that he would quintuple the U.S. target for biofuels: by 2017 they should be supplying 24% of the nation's transport fuel.

So what's wrong with these programs? Only that they are a formula for environmental and humanitarian disaster. In 2004 I warned, on these pages, that biofuels would set up a competition for food between cars and people. The people would necessarily lose: those who can afford to drive are richer than those who are in danger of starvation. It would also lead to the destruction of rainforests and other important habitats. I received more abuse than I've had for any other column - except for when I attacked the 9/11 conspiracists. I was told my claims were ridiculous, laughable, impossible. Well in one respect I was wrong. I thought these effects wouldn't materialize for many years. They are happening already.