"But looking at emissions at the outset, it really depends on how the ethanol is produced," he cautioned governments seeking to tap into biofuels to reduce the greenhouse gas emissions that are blamed for global warming.

Mark Strutt, a Greenpeace campaign coordinator in London, said of the pros and cons of biofuels: "It's much more difficult to define the benefits to the environment if a global market develops."

Corn-based ethanol from the United States and ethanol distilled from Brazilian sugar make up nearly all the world's ethanol production.

Pollution comes from fertilisers and pesticides used to grow crops for ethanol plants, and from fossil fuels burnt during manufacture and shipment, Santillo said.

"Some parts of the US, for example, have plants creating ethanol but they are burning dirty fossil fuels to do so."

Bryony Worthington, London climate campaigner for Friends of the Earth, said: "The impact on the climate depends on how it's grown, where it's grown and where the market is."

Global ethanol production is expected to rise again this year after hitting a record 41 billion litres in 2004 as the 135 signatories to the Kyoto Protocol try to stem global warming by burning less fossil fuel.

The EU, for example, has recommended its member states use vehicle fuel with 5.75 percent ethanol content by 2010.

The United States may not have signed the global Kyoto environmental pact, but is another leader in the drive towards renewable energy.

The Renewable Fuels Association (RFA), the US ethanol industry lobby group, says there is a clear environmental advantage to ethanol fuel.

RFA spokesman Monte Shaw says ethanol use in vehicle fuels in the United States prevented 6.38 million tonnes of carbon dioxide getting into the atmosphere in 2004.