Advances in gene therapy means sportsmen could re-engineer their bodies for success - and never get caught.

Doctors are working hard to develop gene therapies for a host of diseases such as cystic fibrosis and even cancer.

They are hoping to use viruses to carry genetic material into human cells and alter the way they work.

But reports New Scientist magazine, another part of medical science is developing "gene doping", techniques which could give athletes an illegal edge, but which are also virtually undetectable.

However, the quest for more endurance or bigger muscles could ruin the athlete's health in later life.

More stamina

Most athletes naturally have some genetic characteristics which improve their chances of success, even if it is only their height and build.

But some have extraordinary talents - Finnish cross-country skier Eero Mantyranta, who won two gold medals at the 1964 Winter Olympics, had a naturally-occurring genetic mutation which improved the number of red blood cells in his bloodstream.

Mantyranta's mutation meant that the body's mechanism for regulating the number of red blood cells was wrongly set up, leading to the constant production of red blood cells.

Some athletes have abused the key hormone in this process, called erythropoeitin, or epo, yielding vast gains in endurance.

However, gene therapy may allow athletes to reproduce Mantyranta's gene mutation with a single injection.

It has been tested on monkeys with striking success - and may legitimately help anaemia patients in hospitals.

Muscle builder

Another advance causing concern in the sporting world is insulin-like growth factor 1, which plays a role in the body's natural muscle repair processes.

Doctors are hoping to use gene therapy using this to treat muscle-wasting illesses such as muscular dystrophy.

However, IGF-1 can be injected into specific muscles and increase muscle mass, with obvious attraction in certain sports.

It is relatively safe because it is metabolised close to the injection site, meaning there is little danger of the heart becoming dangerously enlarged.

The only hope that the sporting authorities have of catching future gene cheats is to look for the traces of the virus used to carry the genes into the body.

But experts admit this is like looking for a needle-mark in a haystack.