Sepeng, who won the silver medal at the Atlanta Olympics nine years ago, was tested out of season in his home town of Potchefstroom, north west of Johannesburg, in February.

Sepeng, who was not available to comment, has until Monday to explain to the ASA how the substance came to be in his system and whether or not he wants his 'B' sample to be tested.

Norandrosterone is the metabolite of the steroid nandrolone, which shows up in a urine sample.

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Dr Ismail Jakoet, a member of the World Anti Doping Agency (WADA) observer team to the 2001 World Athletics Championships, offered a glimmer of hope for the 30-year-old athlete.

"There have been hundreds of cases where athletes have escaped a ban because nandrolone has been found in contaminated supplements," he told Reuters.

Springbok rugby player Cobus Visagie won a case in 2001 when he proved his body naturally produced excessive levels of nandrolone under stressful conditions.