Some oil representatives have told the government that they cannot meet the UK target of 5% biofuel on the forecourt by 2010 while also protecting wildlife.

The EU recently announced plans to double the biofuel total by 2020.

One government official told the BBC: "The policy is running ahead of the science; we have to be very careful that this doesn't all go badly wrong."

Experts agree it makes sense to maximise wood waste and to grow energy crops on land that is marginally productive for food.

Clean energy or grim reaper? Quick guide: Biofuels

The problem is the scale of the enterprise. Many biologists warn there is simply not enough land on the planet to feed a growing number of people who are hungry for more protein, but also want to run cars on fuel from plants.