"overambitious" and an "experiment" whose "unintended effects are difficult to predict and difficult to control."

"The idea was that we felt we needed to slow down, to analyze the issue carefully and then come back at the problem," Laszlo Somlyody, the panel's chairman and a professor at the Budapest University of Technology and Economics, said in a telephone interview.

He said that part of the problem was that when it set the targets, the European Union was trying desperately to solve the problem of rising transportation emissions "in isolation," without adequately studying the effects of other sectors like land use and food supply.

"The starting point was correct: I'm happy that the European Union took the lead in cutting greenhouse gasses and we need to control traffic emissions," Somlyody said. "But the basic problem is it thought of transport alone, without considering all these other effects. And we don't understand those very well yet."

The panel's advice is not binding and it is not clear whether the European Commission will follow the recommendation.

It has become increasingly clear that the global pursuit of biofuels - encouraged by a rash of targets and subsides in both Europe and the United States - has not produced the desired effect.

Investigations have shown, for example, rain forests and peat swamp are being cleared to make way for biofuel plantations, a process that produces more emissions than the biofuels can save. Equally concerning, land needed to produce food for people to eat is planted with more profitable biofuel crops, and water is diverted from the drinking supply.