The study suggested scientists and farmers focused on crops, which required less intensive farming methods, to produce better benefits for the environment.

Biofuels are derived from plants which absorb the planet-warming greenhouse gas carbon dioxide as they grow, and so are meant as a climate-friendly alternative to fossil fuels.

But the new study shows that some biofuels actually release more greenhouse gases than they save, because of the fertiliser used in modern farming practices.

The problem greenhouse gas, nitrous oxide, is more famous as the dentists' anaesthetic "laughing gas," and is about 300 times more insulating than the commonest man-made greenhouse gas carbon dioxide.

"The nitrous oxide emission on its own can cancel out the overall benefit," co-author Professor Keith Smith told Reuters in a phone interview.

The results, published in "Atmospheric Chemistry and Physics Discussions," were based on the finding that fertiliser use on farms was responsible for three to five times more such greenhouse gas emissions than previously thought. http://www.atmos-chem-phys-discuss.net/7/11191/2007/acpd-7-11191-2007.pdf

They cast further doubts on the credibility of biofuels as a climate cure, following the revelation of other unintended side effects such as rainforest clearance and raised food prices, from competition with forests and food for land. Brazil and the United States produce most of the world's bioethanol, as a substitute for gasoline, while the European Union is the main supplier of biodiesel.

"FUTILE EXERCISE"

Using biodiesel derived from rapeseed would produce between 1 and 1.7 times more greenhouse gas than using conventional diesel, the study estimated.

Biofuels derived from sugar cane, as in Brazil, fared better, producing between 0.5 and 0.9 times as much greenhouse gases as gasoline, it found.

Maize is the main biofuels feedstock used in the United States, and produced between 0.9 and 1.5 times the global warming effect of conventional gasoline, it said.

"As it's used at the moment, bioethanol from maize seems to be a pretty futile exercise," Smith said.

The study did not account for the extra global warming effect of burning fossil fuels in biofuel manufacture, or for the planet-cooling effect of using biofuel by-products as a substitute for coal in electricity generation.

"Even if somebody decides that our numbers are too big ... if you add together the undoubted amount of nitrous oxide that is formed, plus the fossil fuel usage, with most of the biofuels of today you are not going to get any benefit," Smith said.

However, the study did not condemn all biofuels, suggesting that scientists and farmers should focus on crops needing little fertiliser, and harvesting methods that were not energy intensive.

"In future if you use low nitrogen demanding crops, and low impact agriculture, then we could get a benefit," Smith said.