A confidential directive sent by the World Anti-Doping Agency (WADA) to their accredited testing laboratories, a copy of which has been seen by The Sunday Telegraph, warns that the recent discovery of a phenomenon known as "unstable urine" could invalidate a positive finding for the muscle-building steroid - one of the most commonly detected banned drugs in sport.

The agency admit there is now evidence nandrolone, or its related compounds 19-NA and 19-NE, can form spontaneously after an athlete's urine sample has been taken away for testing. Research is still being carried out to try to explain the chemical reaction but it could be the result of bacterial degradation of naturally occurring hormones.

In the meantime, WADA have instructed laboratories to carry out "stability tests" on urine samples that have a high density, an indicator of instability, and low concentrations of 19-NA and 19-NE. They have been asked to report any relevant cases immediately.

The agency have also taken the unprecedented step of raising the threshold for a positive nandrolone finding from two to 10 nanograms per millilitre of urine in samples shown to be unstable. Previously, the 2ng limit was set in stone and, under the strict liability rule, any nandrolone reading above this level was automatically recorded as a positive drug test, whatever the athlete's explanation. Although WADA believe instances of unstable urine are "rare," they admit several new cases have recently come to light. If their laboratories discover more examples, the credibility of the agency's testing procedures could be under threat.

Simon Davis, a British authority on dope-testing, said he believed the new findings could have a bearing on as many as 70 per cent of positive nandrolone cases. He said: "Further research is undoubtedly needed but if the leaked document is correct, at a minimum all nandrolone positives with high sg's [specific gravity, or density] and two to 10ng concentration must be rescinded as a matter of urgency."

The latest discovery revives memories of the case of British middle-distance runner Diane Modahl, who proved that high levels of testosterone detected in her sample in the mid-Nineties were the result of a bacterial reaction due to poor refrigeration of her sample container. Since nandrolone came to prominence in the late Nineties, numerous high-profile athletes have been banned for testing positive for low levels of the steroid. Among the first was Britain's Dougie Walker, a former European 200 metres champion who served a two-year ban, and 400m runner Mark Richardson, whose suspension was cut to a year after he proved that supplements he had been taking had been contaminated with the drug.

More recently, Greg Rusedski and seven other professional tennis players recorded nandrolone levels fractionally above 2ng but well below 10ng. All were subsequently cleared by a tribunal after successfully arguing that contaminated electrolytes handed out by trainers working for the Association of Tennis Professionals were to blame.

WADA strongly criticised the ATP and said the players' exoneration was "greatly disturbing" because it undermined the principle of strict liability, whereby an athlete is solely responsible for what he or she consumes. Ironically, the new research findings have now forced the agency to accept nandrolone levels even higher than those recorded by the tennis players, blowing another hole in their zero-tolerance wall.

For some athletes, however, the scientific breakthrough is too late. Walker, who always protested his innocence, has now retired from athletics and declined to discuss his case, though it is understood the sample he provided in 1998 showed some of the typical features now associated with unstable urine.

But it is unlikely he will have any legal recourse since any athlete seeking compensation would almost certainly have to prove that their urine sample was undeniably unstable - something that would be impossible since instability tests were not carried out at the time.

Sports lawyer Nick Bitel, who represented Walker during his disciplinary hearings, said: "My real issue with the anti-doping system is that it is based on the scientific knowledge of the day and no matter what the experts say now, they will find new things out later. Nandrolone is one of the best examples of this.

"Many years ago they said it didn't occur naturally in people, full-stop. Then they said it occurred only in pregnant women. Then they said it occurred in small quantities in men. What worries me is that we are classifying people as cheats based on scientific knowledge as it is at the moment, and this is what I think happened to Dougie Walker. He was a great athlete who had his career truncated through no fault of his own. It is very sad."
(The Telegraph: www.telegraph.co.uk)