Professor Robert Watson called for the move - designed to cut harmful carbon emissions - to be postponed while a review of the impact of biofuels is carried out.

The Renewable Transport Fuels Obligation (RTFO) is due to take effect from April 1, when biofuels will have to comprise at least 2.5% of fuel at the pumps.

But Prof Watson, chief scientist at the Department for the Environment, cautioned that there were still doubts about the effects of at least some biofuels. "Many people still believe that some biofuels are indeed still sustainable from an environmental standpoint," he told BBC Radio 4's Today programme.

"I think we need to check that. It would obviously be totally insane if we had a policy to try and reduce greenhouse gas emissions through the use of biofuels that's actually leading to an increase in the greenhouse gases from biofuels."

Biofuels, mainly ethanol and diesel made from plants, have been promoted as an alternative to the use of liquid fuels in transport which account for about a quarter of global greenhouse emissions.

A coalition of Britain's leading environmental and development groups has written a joint letter to the Government warning that its biofuels policy risked doing more harm than good.

In a letter to Transport Secretary Ruth Kelly, groups including Oxfam, CAFOD, RSPB, IIED, Friends of the Earth and Greenpeace said "there is a very real risk that the RTFO will make climate change worse, not better".

Doug Parr, chief scientific adviser of Greenpeace, said: "From next month British motorists will be forced to pump biofuels into their tanks with no way of knowing if the so-called green fuels they're using are actually worse for the climate than regular fossil fuels.

"For one of the Government's top scientists to describe these plans as potentially insane suggests that something has gone seriously wrong here."