The Beijing Olympics may be plagued by an undetectable form of doping, a leading British scientist has warned today.

This month's Games are believed to feature a new generation of "genetically modified" athletes whose performance has been improved by the injection of foreign DNA.

It could be the first Olympics to be tainted by "gene doping", the latest and most dangerous threat to drug-free sport.

The process, placed on the list of banned substances and methods by the World Anti-Doping Agency in 2003, involves genes being inserted into muscle or bone cells and their proteins fed directly into the tissue or red blood cells.

This is usually done by injecting, or sometimes inhaling, the necessary DNA.

Dr Andy Miah, in Beijing conducting research during the Games, said: "Gene doping is the next major headache for the world of sport.

"In 2004, people were starting to talk about its use at the Athens Olympics. This year in Beijing, the case is even stronger that this will be the first genetically modified Games. Many scientists will say it's still not possible, but I'm not taking this for granted. We need to assume that it's happening. It's already feasible."

Dr Miah's allegations will cast yet another unwelcome shadow over preparations for the 29th modern Olympics, which begin on Friday. He added: "There is no other technology that is likely to change the Olympics [more] than gene doping. It's not possible to detect and there's a good chance that it will never be detectable in any meaningful sense.

"This forces the world of sport to reconsider what it does about testing. It's time for their plans to change. It's time for the era of human enhancement to take full effect in the Olympics"

Gene doping, to date, has been considered by most experts to be a hypothetical threat.

Many believed it would not register on the Olympic radar until at least the 2012 Games in London.

Dr Miah's claims will cause the WADA and the International Olympic Committee serious concern.

Dr Miah is a researcher in bioethics at the University of the West of Scotland and the Institute for Ethics and Emerging Technologies in the US. He is also the author of several books on the subject. He added: "London 2012 should be watching Beijing very carefully to see what's possible. There has never been a 'clean' Olympics.

"The main problem for sports is that there are so many technologies that are under the radar of anti-doping, that its policies do little more than to point us to successes of anti-doping testing."

This development comes just weeks after a documentary revealed that a hospital doctor in China was prepared to give illegal performance-enhancing gene therapy treatment to an Olympic swimmer.

The doctor was caught on camera by a German television investigator saying that he wanted £12,000 for a two-week treatment that would help to strengthen the fictitious American swimmer's lungs.

The documentary, broadcast by ARD on Germany's main channel this month, showed the head of a hospital gene therapy department being approached by a fictitious swimming coach seeking stem-cell treatment.

The doctor replied: "Yes. We have no experience with sports people here, but the treatment is safe and we can help you."

Asked how it would work, the doctor said: "It strengthens lung function and stem cells go into the bloodstream and reach the organs. It takes two weeks.

"I recommend four intravenous injections ... 40 million stem cells or double that, the more the better. We also use human growth hormones, but you have to be careful because they are on the doping list."

Frederic Donze, of WADA, told the Evening Standard in Beijing today: "We have been preparing for gene doping since 2002.

"We have to believe that athletes will try anything to get an edge and this might occur at the Olympics and we work on that basis."

He said that the WADA had called together experts at a meeting in St Petersburg recently to draw up plans to combat the new drugs menace.