Several international-level athletes have approached leading American scientists and asked them to modify their bodies genetically to turn them into unbeatable sportsmen.

The revelation has stunned sporting organisations, for it shows sportsmen are already trying to use ground-breaking genetics techniques - aimed at saving patients' lives by altering the make-up of their DNA - to boost physique and stamina.

Olympic authorities said yesterday that they feared genetically-enhanced sprinters, swimmers and other contestants will become commonplace. There is even a risk that a few may compete - undetected - at next month's Olympic Games in Athens.

'We cannot be 100 per cent sure, because there are no tests as yet to discover whether an athlete has undergone gene therapy,' said Frederic Donze of the World Anti-Doping Agency (Wada), part of the International Olympic Committee. 'We know some athletes are ready to do anything to cheat and to submit their bodies to high risks, and therefore we assume that at least some athletes would be prepared to try this.'

The controversy has been heightened by the admission of Lee Sweeney, a leading gene therapist, that several athletes had recently asked him to use the technique in order to give them more power and speed.

Sweeney, a professor of physiology at Pennsylvania University School of Medicine, has refused to name them, but has admitted some coaches had also offered their runners as guinea pigs for his experiments, even though his research has so far only involved mice. One coach even offered his entire high-school football team.

Donze commented: 'These people were prepared to try anything that would help them cheat, even though gene transfer has not even yet evolved to the stage where it is used for therapeutic purposes.'

The problem is highlighted by Sweeney, who is developing a method to boost muscle growth among sufferers of the wasting illness muscular dystrophy, in an article in the current issue of Scientific American. 'This kind of gene therapy could transform the lives of the elderly and people with muscular dystrophy,' he says.

'Unfortunately it is also a dream come true for an athlete bent on doping. The chemicals are indistinguishable from their natural counterparts and are only generated locally in the muscle tissue. Nothing enters the bloodstream, so officials will have nothing to detect in a blood or urine test.'

Sweeney's work has involved engineering a harmless virus to carry a gene known as insulin-like growth factor 1 (IGF-1) which is vital for muscle growth and repair. Injected into healthy mice, the virus carried the gene into the animals' cells, where it was incorporated in their DNA. The muscles of the mice grew in size and strength by up to 30 per cent.

In one experiment, Sweeney's team injected IGF-1 into the leg of a rat which had to climb ladders to reach its food. At the end of the experiment, the gene-doped leg was nearly twice as powerful as an untreated one. It also retained its strength for longer after the end of the experiment.

Such experiments are at the cutting edge of genetics. In the past, scientists have isolated human genes and inserted them into bacteria which have then been grown in vats. In this way, it has been possible to produce human proteins that were previously unavailable to doctors.

An example is erythropoietin (EPO) which boosts red blood cell growth. It has provided anaemia sufferers with a lifeline but has also been exploited by distance runners and cyclists seeking to boost their oxygen uptake. The Scottish cyclist David Millar was banned this month from the Olympics after admitting taking EPO.

These drugs are detectable in the blood. But by taking genes and directly injecting them into people, the problem is avoided. But there is severe danger involved in such techniques. Gene therapy has been known to trigger fatal immunological side-effects. In experiments on monkeys and baboons which were genetically manipulated with the EPO, their blood became so thick it had to be regularly diluted to keep their hearts from failing. 'It's a frightening prospect,' said Michele Verroken, former head of anti-doping at UK Sport. 'Athletes are going to be attracted to gene doping because they won't get caught and because they'll see it as safer than drugs, which can lead to addiction and overdose. But we don't know what gene therapy will do to athletes' bodies and what you are producing.'

The allure of gene-doping is considerable, as Professor Ken Fitch, a member of Wada's board, pointed out: 'If you inject it in the take-off muscles of a high jumper, suddenly he's got 50 per cent more power than he ever had. How high is he going to jump? If he's a 2.20-metre high jumper, he might be jumping 2.70m. So the potential is there.'

Opinion is divided about how soon gene doping will start being abused by sportsmen. 'It's highly unlikely that athletes are using gene transfer today. But scientists say that they could be using it by 2008 or later,' said Donze.

But, as Sweeney points out, treatments like his IGF-1 injections could be vital in regenerating torn or injured muscles, and there is no ethical reason why an athlete should not take advantage of that. The authorities are now faced with the prospect of deciding where treatment ends and morally unacceptable enhancement begins.