Mice given the drug burned off fat, even when they did not exercise, and were resistant to weight gain despite a high-fat diet.

The ultimate use would be to treat people at risk of obesity-related diseases like diabetes, rather than offer a "no-work six-pack" pill.

The Salk Institute team presented their work at Experimental Biology 2007.

The drug mimics normal fat and chemically triggers a gene switch called PPAR-delta.

Turning on this switch activates the same fat-burning process that occurs during exercise.

Lead researcher Dr Ronald Evans believes the same will occur in humans.

Human trials

UK expert Dr Fredrik Karpe, from the Oxford Centre for Diabetes, Endocrinology and Metabolism, is hoping to test this in the near future.

Commenting on the work, he said: "There has never been a method to 'medically' switch on fat burning before.

"The finding that PPAR-delta co-ordinates this process, not only by switching on fat burning, but also to rebuild the muscle in a way making it more fit for fat burning, is of major interest, not least as a completely novel approach for the treatment of the metabolic derangements accompanying obesity."

But he cautioned; "Although this might become an 'exercise pill', it is unlikely to provide all the other benefits of real physical exercise."