THE very credibility of athletics was under scrutiny last night after a rash of renowned figures were found to have failed drugs tests just a month before the 2013 World Championship begins in Moscow.

The sport's showpiece competition, and especially the blue riband men's 100m, will be held under a cloud after it emerged yesterday that first Tyson Gay, then Asafa Powell, were found to have tested positive for banned substances. American Gay, the second fastest man in history behind Usain Bolt and considered his main rival for the world title at both 100 and 200m given his position at the top of the rankings this season, was last night waiting for the results of his B sample from the US Anti-Doping Agency but has reportedly already withdrawn from his national team.

Former 100m world-record holder and Olympic 4x100m gold medalist Powell, meanwhile, tested positive for the same banned stimulant found in a sample belonging to his compatriot Sherone Simpson, herself a three-time Olympic champion. A further three Jamaicans have also reportedly failed tests in the wake of last month's national championships. Usain Bolt is not among them.

Powell, who held the world record before Bolt, last night released a statement in which he insisted he had not knowingly doped. The 30-year-old, who has run 9.88 seconds this year, but failed to make the Jamaican team for the World Championships, said a sample he gave at the national trials in June returned "adverse findings" for oxilofrine (methylsynephrine).

"I want to be clear that I have never knowingly or wilfully taken any supplements or substances that break any rules," it read. "I am not now – nor have I ever been – a cheat. My team has launched an investigation and we are co-operating with the relevant agencies and law enforcement authorities to discover how the substance got in my system.

"I am reeling from this genuinely surprising result. My fault is not cheating but not being more vigilant. I want to reiterate that I have never sought to enhance my performance with any substance. It is not a part of who I am or what I believe in."

Gay, too, has been vocal in the fight against doping, having participated in a US programme called 'Project Believe', for which he submitted voluntarily to extra testing. The 30-year-old, whose 9.69s in 2009 is second only to Bolt in the all-time rankings, won the sprint double at the 2007 World Championships in Osaka, but has been plagued by injury problems in recent seasons.