Powell confirmed the news in a statement released via his Twitter account in which he denied that he ever knowingly or wilfully took any banned substances.
“I will confirm that a sample I gave at the National Trials in June has returned ‘adverse findings’,” said the 30-year-old.
“The substance oxilofrine was found, which is considered by the authorities to be a banned stimulant.
“I want to be clear 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.”
Powell, who has never won an individual global sprint title, held the 100m world record between 2005 and 2008 when his then-best of 9.74 seconds was broken by current record holder and fellow Jamaican Usain Bolt.
“This result has left me completely devastated in many respects,” Powell said. “Professionally, this finding fully negates any possibility of me being a part of Jamaica’s contingent of athletes competing at [the] world championships in Moscow later this summer.”
Powell, who was battling injuries earlier this year, failed to qualify for next month’s Moscow world championships, finishing seventh in the 100m.
He has since re-emerged as an elite sprinter, clocking 9.88 seconds earlier this month in Lausanne and raising speculation he could be added to Jamaica’s 4x100m relay team.
Powell’s agent, Paul Doyle, said there was no intent by Powell to use banned substances.
“We are working with WADA [World Anti-Doping Agency] now on how it got in his system,” he said.
Powell added: “I accept the consequences that come with this finding — after all there is only one Asafa Powell.
“My fault here, 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.”
Meanwhile Gay, the fastest man to take to the track this year, said that while he would wait for the results of his B sample, he will accept any sanctions handed to him.
“I don’t have a sabotage story... I basically put my trust in someone and was let down,” he told the BBC.
The news comes just a month after Jamaica’s Olympic champion Veronica Campbell-Brown tested positive for a banned diuretic.