A BBC investigation has found that there are serious question marks over a key drug test just two weeks before the start of the Beijing Olympics.
The BBC has seen indications that labs are classing positive tests for the blood-boosting drug EPO as negatives.
Some samples have been described as suspicious - giving rise to fears that no action will be taken against cheats.
One sport drug expert told the BBC that many of the finalists in Olympic endurance events would be using EPO.
"Copycat" versions of the drug are available on the internet for as little as $50 - and according to experts are often undetectable.
Although a test was introduced to detect recombinant EPO (erythropoietin) at the Sydney Olympic Games in 2000, a growing number of athletes were soon challenging the results in the courts.
Several, like US sprinter Marion Jones, had their first sample test positive but were cleared on the second or B test.
I would think that most of the medal winners and many in the finals of endurance events - there is a big risk for them having used EPO
Professor Bengt Saltin
In response, in 2004, the World Anti-Doping Agency (WADA) tightened the criteria by which an EPO positive could be declared.
As a result, the number of legal challenges fell. But according to experts we have spoken to, athletes continued to abuse EPO. As they became more adept at medicating themselves, so the number of positives declined.
Many scientists blame the anti-doping laboratories for a loss of nerve in the face of continuing EPO use.
Dr Rasmus Damsgaard runs the anti-doping programme for the International Ski Federation and for the Astana Cycling team. He is widely admired for his innovation and energy. He says he has clear evidence that positive EPO tests are being declared as negative or suspicious.
Earlier this year, he sent five samples from skiers to a WADA lab for analysis. They all came back negative. But when Dr Damsgaard demanded the gels or electronic printouts on which the determination of guilt or innocence was made, he was astonished to see what he believed was clear evidence of EPO use.
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"It was very obvious that the gels were very un-natural or very different from natural distributions. But I also saw that they were declared negative because they didn't fulfil the WADA criteria of a positive test; although they looked suspicious and had no natural bands at all, they were still declared negative."
And Dr Damsgaard believes that there are many more such samples in WADA labs.
"From a little work with a lot of blood profiles, I found maybe five positives. I wonder that maybe hundreds, maybe even thousands of EPO positive samples are lying around in WADA-accredited labs."
He is not alone in that view. Professor Bengt Saltin is a leading anti-doping expert and a former winner of the IOC Olympic Prize, the highest honour in sports science.
He says that the new WADA criteria allow most EPO cheats to get away with it.
I can assure you that if you were to take recombinant EPO and that would be in your urine - then, yes, we would detect it
Dr Olivier Rabin, WADA
"The reason that I am still a little bit upset with the whole situation is that I have seen too many suspicious samples that are clearly abnormal. Athletes are getting away with it. Look how many have been caught for EPO misuse recently."
In fact, the numbers of athletes being prosecuted for EPO use has declined significantly - by two-thirds between 2003 and 2006.
Dr Olivier Rabin is WADA's science director. Is he happy that the test is catching all the drug cheats?
"I am reasonably confident, yes," he told the BBC. "Now, it would be very presumptuous on my part to say that we are absolutely 100% sure we are going to get everyone. But I can assure you that if you were to take recombinant EPO and that would be in your urine - then, yes, we would detect it."
But the BBC's sources are highly critical of the performance of the WADA-accredited laboratories that carry out EPO tests.
Danish researchers recently set out to test how well the labs can detect EPO by medicating eight student volunteers with the drug over a period of weeks. The athletic performance of these students improved in some cases by 50%. But when over 100 urine samples from the students were sent anonymously to two WADA-approved laboratories, they produced very different results. One of the labs declared none of the samples positive.
Rasmus Damsgaard says the current testing regime is fatally flawed.
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"WADA is sitting on a mountain of positive EPO. They have these very strict rules, and declare that everything is working fine. But it's not working at all! You can more or less do whatever you like with EPO and you will not be charged."
There is also concern that scientists in some WADA labs may be actively colluding with athletes or their medical advisers. Senior academics are currently investigating possible collusion between doctors who were working with cycling teams and WADA-accredited labs. They are worried that these doctors were gaining inside information on the latest test procedures.
Former professional cyclist Jorg Jacksche admitted that he had been a long-time doper when he was caught up in a drugs ring in Spain in 2006. He believes that collusion between labs and the doctors working with athletes is a fact of life:
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"There must have been also a certain knowledge exchange between cycling team doctors and persons that are working in the laboratories - otherwise how would they know? It's a science of doping, and there is a knowledge exchange between team doctors who are close to anti-doping institutions - you cannot do it by trial and error, there would have been too many positive cases."
Another growing problem is the rise of copycat versions of EPO.
Because the medicine has been so successful financially, companies in India, China and Cuba have developed drugs that do a similar job in the body, but have a slightly different molecular fingerprint. These cheap versions of EPO, often called biosimilars, can be easily bought over the internet.
Professor Werner Franke first became known when he helped uncover the systematic doping practices of the East German state. He says that this is a growing problem.
"There are now a number of compounds that bind to the EPO receptor, and there is no single test for them, you can order it here over the internet and it will be delivered to you here in the UK or in Germany; Chinese-made doping substances"
Some scientists who track and monitor the development of copycat EPO drugs say there could be up to 80 different versions now being manufactured in different parts of the world.
Experts that we have spoken to say that WADA should widen its criteria in declaring a positive.
They say that anti-doping officials should determine if a sample has any evidence of naturally produced EPO. If it hasn't, it should be classed as suspicious because the use of artificial EPO for doping causes the body's own production to shut down.
The scientists also argue that the tests should examine blood profiles as well as the urine sample. An analysis of the number of young red blood cells can also indicate doping.
While that may happen in the future, it will not prevent cheating at Beijing. Bengt Saltin says he has never been more depressed about the use of this drug.
"I would think that most of the medal winners and many in the finals of endurance events - there is a big risk for them having used EPO. Of course, they have to be clever but they don't have to be very clever."
RESULTS OF EPO TESTING
The left hand graph shows the blood profile of an athlete who has not taken EPO. The urine test shows a grouping of white markers which represent a normal level of EPO in the blood.
The right hand graph shows the blood profile of an athlete who has taken EPO. It shows a higher-than-normal proportion of reticulocytes, the young red blood cells produced by the bone marrow, due to stimulation by EPO.
The positive urine test shows a lack of normal EPO - indicated by fewer white markers - suggesting the body has stopped producing its own EPO.
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