Tyson Gay is waiting for the results of his B sample from the US Anti-Doping Agency

The athletics world was rocked to its very core tonight by the biggest drug scandal to hit the sport in years.

Jamaican sprinters Asafa Powell and Sherone Simpson tested positive for the same banned stimulant, while Tyson Gay was waiting for the results of his B sample from the US Anti-Doping Agency amid reports he too had failed a drug test.

Powell, the former 100 metres world record holder, and American Gay, the second fastest man in history, are two of the biggest names in athletics, which is now facing up to its biggest crisis of credibility since the BALCO scandal.

Simpson is a three-time Olympic medallist, while three more of their compatriots also failing tests, according to the Jamaican Gleaner newspaper. Usain Bolt is not among them.

Powell, who has won gold medals in the 4x100m relay at the Olympics and World Championships and held the world record before Bolt, released a lengthy 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 next month's World Championships, said a sample he gave at the national trials in June returned "adverse findings" for oxilofrine (methylsynephrine).

He said: "I want to be clear in saying to my family, friends, and most of all my fans worldwide that I have never knowingly or wilfully taken any supplements or substances that break any rules.

"I am not now - nor have I ever been - a cheat.

"My team has launched an internal investigation and we are co-operating with the relevant agencies and law enforcement authorities to discover how the substance got in my system."

Powell ran his world record of 9.72sec in 2008 and has often failed to live up to expectations on the biggest stage.

He added: "My attitude towards doping regulations and testing is well-known and I willingly give samples whenever requested.

"This result has left me completely devastated in many respects.

"Professionally, this finding fully negates any possibility of me being apart of Jamaica's contingent of athletes competing at the World Championships in Moscow later this summer.

"Personally, however, this result comes at a greater cost. I write this statement knowing fully that my family, friends, fans and country will be disappointed in this latest development.

"I am reeling from this genuinely surprising result. I am confident, however, that I will come out stronger and wiser and better prepared to deal with the twists and turns of being a professional athlete.

"I accept the consequences that come with this finding - after all there is only one Asafa Powell.

"My fault however is not cheating but instead not being more vigilant.

"I want to reiterate that in my entire career as an athlete 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."

Simpson, who won 4x100m relay silver at London 2012 and individual silver in Beijing four years earlier, tested positive for the same substance, Paul Doyle, agent to both athletes, confirmed in an email to Press Association Sport.

Bolt's agent Ricky Simms said the world's fastest man was not caught up in the scandal, saying: "I was just with him in Jamaica and we've had no information and we've not been contacted by anyone. It is not anyone that I'm connected with."

The news of Powell and Simpson's failed tests broke just hours after Gay was reported to have revealed he had been informed by USADA that an A sample from an out-of-competition test had come back positive.

A USADA statement read: "USADA appreciates his approach to handling this situation and his choice to voluntarily remove himself from competition while the full facts surrounding his test are evaluated.

"The B sample will be processed shortly, and as in all cases all athletes are innocent unless or until proven otherwise through the established legal process, and any attempt to sensationalise or speculate is a disservice to due process, fair play, and to those who love clean sport."

Gay has reportedly pulled out of the American team for next month's World Championships.

The American's showdown with Bolt over the 100 and 200m was set to be the highlight in the Russian capital, with Gay topping the 100m world rankings this year.

Commenting on the case, USA Track and Field chief executive Max Siegel said: "USA Track and Field is strongly opposed to doping, and we respect the work that USADA has done as a leading agency globally in the fight against drugs in sport.

"We do not know the facts of this case and look to USADA to adjudicate it and handle it appropriately. It is not the news anyone wanted to hear, at any time, about any athlete. As we approach the World Championships, we will remain focused on the competition at hand and winning the right way."

Gay has been involved in the fight against doping, having participated in a US anti-doping programme called 'Project Believe', for which he submitted voluntarily to extra testing.

The 30-year-old, whose 9.69s clocked 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.

He has been back to full fitness this year, though, and led the rankings with 9.75. Indeed, he has run the three fastest times of the year, with Bolt's best 9.94.

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