A "gene doping" technique that makes the muscles of mice grow up to 30 per cent stronger will be the next frontier of cheating in professional sport, scientists gave warning today.

The new form of gene therapy, which is being developed to treat patients with muscular dystrophy and other wasting diseases, will inevitably be abused by athletes seeking the ultimate illegal edge, according to the researchers behind it and anti-doping experts.

Experiments at the University of Pennsylvania have revealed that when normal mice are injected with a modified virus that adds a vital growth gene to their cells, they develop into heavily muscled, super-strong animals that the scientists have nicknamed "mighty mice".

The effects are almost doubled when the gene therapy is coupled with a weight-training regime.

Such genetic enhancements would deliver vast improvements in speed, power and strength if given to an athlete, said Lee Sweeney, who led the Pennsylvania research.

Gene doping has the potential to boost an unscrupulous sportsman's performance more steeply even than existing drugs such as anabolic steroids and erythropoeitin (EPO).

It would also be extremely difficult to detect, as it mimics the quirks of inheritance that give some athletes a natural genetic advantage.

Dr Sweeney told the American Association for the Advancement of Science conference in Seattle that while the technology was designed for bona fide medical purposes, it was "inevitable" that it would eventually be abused "off label" in sport.

He has already been approached by coaches and athletes, mainly bodybuilders, seeking to try gene therapy, even though the technique is barely ready for patient trials.

"This is but one example of a number of potential gene therapies that are being developed with disease treatment as the goal, but if given to a healthy individual would provide genetic enhancement of some trait," Dr Sweeney said.

"As these developments go forward, they inevitably will find their way into the healthy population.

"The prospects are especially high that muscle-directed gene transfer will be used by the athletic community for performance enhancement, just as many drugs are used and abused today.