The Doug Walker case, which is to be examined by a UK Athletics committee, is the latest to call into question the whole testing procedure.

Scottish sprinter Walker insists he has never taken any banned performance-enhancing substance.

Sale athlete Diane Modahl made the same claim after a laboratory in Lisbon recorded a positive test by her in 1992.

She was subsequently banned for four years, but had the suspension lifted after a lengthy legal fight to clear her name ended with the Lisbon testing facilities being declared inadequate.

Modahl is suing the British Athletics Federation for more than �500,000, but her case has yet to reach a conclusion as the BAF went into administration in October 1997.

Jason Livingston: Back on the track

Sprinter Jason Livingston served a four-year ban after testing positive for steroids, the case breaking during the 1992 Olympics.

Shot-putter Paul Edwards received a similar suspension in 1994 after failing two separate tests. He returned to competition but failed another test last year and was banned for life.

Solomon Wariso, then British 200 metres champion, tested positive for ephedrine in 1994 and was suspended for three months. He protested his innocence and said he had taken a herbal remedy called 'Up Your Gas'.

The same year saw javelin thrower Colin Mackenzie given a three-month ban after failing a test. He insisted he had taken a pain-killer containing a banned substance.

But all drugs cases are overshadowed by that involving Canadian sprinter Ben Johnson.

He won the 100 metres at the 1988 Seoul Olympics in a world record time but was later stripped of his gold medal and banned for two years after testing positive for an anabolic steroid.

Ben Johnson: The most famous drugs case of them all

A life ban from the track followed in March 1993 when he gave another positive test.

American Harry 'Butch' Reynolds was the 400 metres world record-holder when he was given a two-year ban in 1990, but succeeded in getting his suspension lifted after only eight months.

Reynolds took legal action to claim �18m damages but a court threw out his case.

German athletes Katrin Krabbe, Grit Breuer and Silke Moeller provided identical urine samples at a training camp in South Africa but were cleared on a technicality.

Krabbe and Breuer were subsequently banned for one year each after failing another test.

World athletics officials are currently considering the case of Sri Lankan sprinter Susantika Jayasinghe, who tested positive for the steroid nandrolone in April 1998.

The 1997 world 200m silver medallist is training in the USA.

Colombo's Daily News claims the International Amateur Athletic Federation is recommending a two-year ban from the date of the test.