Anabolic steroids such as THG have been in demand in gyms and sports grounds for decades, and one study found that 9 per cent of body builders going to gyms in Britain were using them.

Made from the male hormone testosterone, they provide a chemical shortcut to strength and endurance. They promote the development of muscle, reduce fatigue and speed recovery after physical exertion by stimulating the production of protein. This makes them especially attractive to sprinters, weightlifters and throwers such as shot putters, for whom raw power is all-important.

The addition of four hydrogen atoms was all it took to make the anabolic steroid, gestrinone, undetectable by standard tests. A clever bit of work by chemists transformed it into tetrahydro-gestrinone (THG), providing some sportsmen, apparently, with the means to cheat.

The alarm was raised when a used syringe with a barely visible residue inside it was provided by an anonymous track and field coach to the US Anti-Doping Agency. From that residue, a University of California laboratory was able to identify the droplets as THG and then devise a test that would detect it in athletes' urine.

UK Sport said yesterday the test for THG would be introduced in Britain as soon as possible. About 7,500 random drug tests are done each year by sport's governing body, of which 2 per cent are positive.

Anabolic steroids carry many risks. They can harm the liver and may damage the heart by causing expansion of the cardiac muscle. They also promote growth of the bones, especially facial bones such as the jaw and teeth, and increase the risk of cancer.

One problem with detecting the use of anabolic steroids is that they are broken down and excreted by the body within five to 15 days but their effects are felt for three to four months.

Athletes may choose to use them during training and then stop a couple of weeks before a competition. Drug testing out of competition and at no notice were introduced to tackle this problem.

In addition to steroids, stimulants such as ephedrine are among the most widely abused drugs in sport. Erythropoietin, used by cyclists and long distance runners to increase the oxygen carrying capacity of the blood, is also in demand.