AMERICAN sprinter Tyson Gay, the fastest man over 100m in the world this year, has tested positive to a banned substance and will not compete in next month's world championships in Moscow.

The 30-year-old has struggled with injuries in the past five years, but returned to his best form to win the US 100m title last month in 9.75secs, and the 200m in 19.74secs.

Gay told the BBC he did not have "a sabotage story".

"I know exactly what went on, but I can't discuss it right now." He said. "I hope I am able to run again, but I will take whatever punishment I get like a man."

A statement by the USADA praised Gay's decision to remove himself from competition until the results of the B sample were available.

Meanwhile Jamaican sprint star Asafa Powell has tested positive to the banned stimulant, oxilofrine.

Powell confirmed via Twitter that the adverse findings came from a test at the national trials in June.

"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," the 30-year-old posted.