A report into drugs in sport has dismissed claims that naturally-occuring nandrolone in food is responsible for the huge jump in positive tests among athletes.

UK Sport's report into nandrolone was commissioned following disputed positive tests of athletes Dougie Walker, Linford Christie and Gary Cadogan.

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The sprinters were all cleared by UK Athletics after they claimed they had not knowingly taken any performance enhancing drugs.

The report rules out poor collection and faulty sample-taking for the 17 positive cases in 1999 but is largely inconclusive.

It says nandrolone can be produced naturally by the body, but adds that further research is needed to assess whether enough can be produced to account for the high levels found in failed cases.

It rules out vegetables such as avocado, which had been discussed as a potential source, and most foods.

Instead, the report points to the growing and unregulated market in dietary and herbal supplements for a possible explanation.

Michelle Verroken, director of ethics and anti-doping at UK Sport, called for manufacturers of the supplements to help set up and finance a voluntary licensing scheme.

"These supplements constitute a huge unregulated market," she said.

"We always tell athletes we cannot give advice on whether they are acceptable to use because they are not subject to any controls so the label may be no indication of what they actually contain."

Observers say UK Sport has been forced to be circumspect over nandrolone because of the possible legal implications.

"UK Sport have had to box clever on this," said Staffordshire University Professor Ellis Cashmore, an expert on drugs in sport.

"On the one hand it does not want to appear too lenient because all these athletes who have tested positive can say they are innocent and you can understand the implications of that.

"On the other hand it does not want to appear so inflexible that it invites a whole series of quite costly legal challenges."

Cheshire-based runner Diane Modahl bankrupted the former governing body of UK athletics with her claim for damages over a disputed drugs test.

And Walker, who was European 200m champion at the time of his test, is threatening to sue UK Athletics, the current governing body, for its failure to sort out his case.

He was cleared of wrongdoing by UK Athletics but the International Amateur Athletics Federation (IAAF) has refused to lift his two-year ban.

Innocence campaign

"Walker has protested his innocence for more than 12 months, added Cashmore.

"Even if he is found to be innocent eventually Walker can say, 'Well I'm 27 next July, you've taken away a whole year while I was at the peak of my career. My earnings would have been possibly millions,' and take them to a civil court.

"That's why this report has been so inconclusive."

The market for nutritional supplements is largely unregulated, but is worth an estimated £3 billion worldwide.

In the past, Christie has backed products such as creatine, while Walker and Cadogan both appeared in a catalogue for a healthfood manufacturer, endorsing their supplements.

Sales of similar supplements in the United States increased by more than 1,000pc during 1998-99, to more than $50m after baseball home run record-breaker Mark McGwire admitted that he was using a supplement that contained androstenedione, another body-building substance on the International Olympic Committee's banned list.