Tyson Gay and Asafa Powell
Gay and Powell and both former world champions.

Tyson Gay and Asafa Powell, the world championship winning sprint pair, have left the reputation of athletics in tatters after both tested positive for banned substances.

Gay, the fastest man in 2013 and joint second in history, was informed by the United States Anti-doping Agency last week that his A-sample, taken during an out of competition test, had returned a positive.

Powell, the Jamaican athlete and part of the 4x100m Olympic gold medal winning team at London 2012 and the 4th fastest man in history, is one of five from his homeland to test positive after last month's Jamaican Championships.

Sherone Simpson, the 4x100m relay silver medallist, is among the remaining quartet also testing positive for oxilofrine.

While both athletes await the results of their B sample, the positive tests cast a shadow over Jamaica's recent sprinting dominance after the country's most successful female sprinter Veronica Campbell-Brown tested positive for a banned diuretic in June.

In a statement, Powell 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 even been -- a cheat.

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

"I assure you that we will find out how this substance passed our rigorous internal checks and balances and design systems to make sure it never happens again."

The American Gay, the world leader over 100m in 2013 after running 9.75 on three occasions, most recently in Lausanne, claims the illegal substances may have entered his system without his knowledge.

"I am not now, nor have I ever been, a cheat," Gay, 30, said.

"I don't have a sabotage story. I don't have lies. I basically put my trust in someone and I was let down.

"I hope I am able to run again, but I will take whatever punishment I get like a man."

Despite Gay pulling out of next month's World Athletics Championships, both athletes face two-year bans from the sport after failing tests for the first time, however, the pair will be hoping to exonerate themselves from blame and have any ban subsequently reduced.

"In response to Mr Gay's statements, 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," Usada said.

"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."

The management representing both Bolt and Yohan Blake have confirmed neither athlete are among the five Jamaican athletes to have tested positive, Herb Elliott, chairman of the Jamaican Anti-Doping Commission says further names will be revealed.

"The process of the result management has commenced in accordance with the JADCO anti-doping rules.

"We cannot disclose any further information until the athletes have responded to notification of the 'A' sample."

Following reports last week that up to 30 Turkish athletes face doping charges - which could lead to the country's exclusion from the World Championships in Moscow - the International Association of Athletics Federations (IAAF) have moved to reaffirm their commitment to tackling doping.

"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," the world governing body said.

"It is for them that we have built a program that is well resourced, far reaching and sophisticated. 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 credibility of our anti-doping program, and the sport of athletics, is enhanced, not diminished, each time we are able to uncover a new case and we have the committed support of every athlete, coach or official who believes in clean sport."