The International Association of Athletics Federations says the credibility of its anti-doping program has been "enhanced, not diminished" after high-profile sprinters Asafa Powell and Tyson Gay returned positive tests for banned substances.

Gay, a former world champion who won the 100- and 200-metres at U.S. nationals last month, said he would pull out of the upcoming world championships.

Powell, who held the world record in the 100 until Usain Bolt lowered it in 2008, and Olympic gold medallist Sherone Simpson also face suspension after failing tests at the Jamaican championships last month.

Suspended Sponsorship

Adidas has suspended its sponsorship of Tyson Gay.

The sportswear giant acted a day after Gay disclosed that a banned substance was detected in an out-of-competition test in May and he had pulled out of next month's world championships in Moscow.

"We are shocked by these recent allegations, and even if we presume his innocence until proven otherwise, our contract with Tyson is currently suspended," Adidas said in a statement.

The 30-year-old Gay, the American record-holder in the 100 metres, has been backed by Adidas since 2005.

"During this time, he has been a great ambassador for the sport of track and field and our brand," Adidas, which is based in Herzogenaurach, Germany, said.

The shoe and sports clothing maker is invoking the clause in Gay's contract relating to doping.

"Adidas has a clear policy on doping and drug use," the company said. "Each of the agreements with our athletes include a clear clause which states that the agreement shall be terminated by Adidas if the athlete is found guilty of the possession or use of drugs or any other prohibited substance by the relevant governing sports body having jurisdiction over the athlete."

— The Associated Press

IAAF spokesman Nick Davies said the governing body for track and field does not comment on pending cases, but added that the fight against doping "is enhanced, not diminished, each time we are able to uncover a new case."

"The IAAF's commitment to anti-doping in athletics is unwavering because we have an ethical obligation to the majority of athletes who believe in clean sport," Davies said in a statement. "The fact that we are able to detect and remove from the sport athletes who have breached our anti-doping rules should be seen in this context."

The sport was thrown into turmoil on Sunday when news emerged of the failed tests by such high-profile athletes.

The 30-year-old Powell called for an investigation into how a stimulant called oxilofrine entered his system.

"I am not now — nor have I ever been — a cheat," Powell posted on Twitter.

Simpson, who tested positive for the same stimulant, said she "would not intentionally take an illegal substance of any form into my system."

The positives recorded by Powell and Simpson are part of a bigger doping crisis hitting Jamaica, the home of Bolt and a country which has dominated sprinting medals at recent Olympics.

In Sunday's editions, The Gleaner newspaper of Jamaica reported that five athletes had tested positive. Paul Doyle, the agent who represents Powell and Simpson, confirmed to the AP that his sprinters were among them. Shortly after Doyle's confirmation, Powell and Simpson each released statements acknowledging the positive tests.

The news came a month after another Jamaican Olympic gold medallist , Veronica Campbell-Brown, tested positive for a banned diuretic.

Campbell-Brown is being suspended while a disciplinary panel reviews her case. She has denied cheating.

'I was let down'

Gay, who won world championships in the 100, 200 and 4x100 relay in 2007, previously took part in the U.S. Anti-Doping Agency's "My Victory" program, in which athletes volunteer for enhanced testing to prove they're clean, and his results never raised red flags until an out-of-competition test on May 16.

"I don't have a sabotage story," Gay said in a telephone interview. "I don't have any lies.

"I don't have anything to say to make this seem like it was a mistake or it was on USADA's hands, someone playing games.

"I don't have any of those stories. I basically put my trust in someone and I was let down."

Neither Gay nor the USADA revealed the banned substance at the centre of the positive sample.