The BBC's George Eykyn: "The system needs an over haul"

Christie, 39, tested positive for traces of the banned steroid, nandrolone, after a contest at Dortmund, in Germany, earlier this year.

The International Amateur Athletic Federation suspended Christie, a strident campaigner for drug-free athletics, who now competes only in selected meetings.

Linford Christie: "I thought it was a joke"

The 1992 Olympic 100m sprint champion told the BBC that suggestions that he uses drugs were "ridiculous".

He said: "When I first heard the news it was a shock. But I'm innocent and I keep telling everyone that.

"At the end of the day I've always been against drugs. I've always said you can get to the top without being on drugs and I just hope that most of the kids out there don't lose heart, because I'll clear my name."

Test confusion

He added: "I'm virtually retired. For me to use drugs at the end of my career is stupid and ridiculous."

BBC Science Correspondent Palab Ghosh: "Some scientists believe not enough research has been done."

UK Athletics, the sport's governing body, has ordered an urgent investigation as calls increased for confusion over nandrolone to be resolved following the case of Scottish sprinter Doug Walker.

Walker, 26, who was suspended from competition in March after failing an out-of-season urine test in December, was cleared last week after a UK Athletics disciplinary committee found there was nothing to prove that the traces of nandrolone had entered his body in the form of an illegal drug.

'Very odd'

Dr Malcolm Brown, director of medical services at UK Athletics, told the BBC: "What's lacking is sufficient scientific research for these findings over the last year.

"There have been a number of reports of athletes around the world having metabolites of nandrolone and this is very odd.

"We need to look at both sides and not jump to conclusions."

Sally Gunnell is backing Christie's fight to clear his name

Fellow athletes including former hurdler Sally Gunnell and world triple jump record holder Jonathan Edwards expressed their disbelief at the result and said Christie was innocent.

Former world 110m hurdles silver medallist Jon Ridgeon said: "After the Dougie Walker thing it's just a joke - a sad joke. I'm sure Linford will be totally exonerated."

But in a fresh blow for Christie, the BBC announced it was reviewing its agreement with the athlete to cover the World Athletics Championship in Seville later this month.

Libel win

Christie was cleared of drug-taking at the 1988 Seoul Olympics when the International Olympic Committee's medical commission accepted that pseudoephendrine had got into his system because Christie had been drinking ginseng tea.

Jayne Pearce from UK Athletics: "It is a complicated area"

Last year he won �40,000 in a libel case over magazine article which alleged he had taken performance-enhancing drugs.

He said on Wednesday that he was prepared to go to court again.

"I spent hundreds of thousands, now lost, to clear my name a few months ago and if needs be I'll do the same again," he said.