Jamaican sprinter Asafa Powell has insisted he is not a cheat and never has been - after he failed a drugs test.
The former 100 metres world record holder admitted in a statement he had tested positive for a banned stimulant.
The 30-year-old said: "I can confirm that a sample I gave at the national trials in June earlier this year has returned 'adverse findings'.
"The substance oxilofrine (methylsynephrine) was found, which is considered by the authorities to be a banned stimulant."
Powell has won gold medals in the 4x100m relay at the Olympics and World Championships and held the world record before his compatriot Usain Bolt.
He has run 9.88 seconds this year, but failed to make the Jamaican team for next month's World Championships.
Powell insisted that he had taken the substance unknowingly.
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.
"I assure you we will find out how this substance passed out rigorous internal checks and balances and design systems to make sure it never happens again."
Powell was reportedly one of five Jamaicans to have tested positive, according to the Jamaican Gleaner newspaper.
But they do not include Bolt, whose agent Ricky Simms said: "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."
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 in his statement: "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.
"I wish to thank all the people who have so far reached out to us to offer support and this experience has reminded me of how blessed and lucky I am to be able to do what I love for a living."
The Jamaica Anti-Doping Commission (JADCo) confirmed to the Gleaner that they had received reports of positive tests.
"The Jamaica Anti-Doping Commission has received reports of adverse analytical findings from A samples," Dr Herb Elliott, the chairman of the JADCo board, was quoted as saying by the newspaper.
"The process of results management has commenced in accordance with JADCo anti-doping rules."
ASAFA POWELL FACTFILE
1982: Born November 23, Spanish Town, Jamaica.
2002: Wins silver as part of Jamaica 4x100m relay team at Commonwealth Games in Manchester. Makes semi-finals in individual 100m with a then personal best time of 10.26 seconds.
2004: Considered favourite for 100m gold at Athens Olympics after recording nine sub-10 second times during the season, including clocking 9.91secs in Jamaican National Championships, but only finishes fifth in the final in 9.94secs. Justin Gatlin won in 9.85secs.
September: Sets new personal best in 100m of 9.87 in Brussels.
2005: June 14 - Breaks 100m world record for first time after running 9.77 seconds in Athens.
2006: March - Wins gold in 100m and 4x100m at Commonwealth Games in Melbourne.
June 11 - Equals 100m world record again in Gateshead.
June 26 - Sets personal best in 200m with 19.90 secs in Kingston.
August 18 - Runs 9.77 once more in Zurich.
November - Named IAAF Athlete of the Year.
2007: August - Wins 100m bronze in World Championships in Osaka and silver in 4x100m at same event.
September 9 - Sets new world record after running 9.74secs in Rieti.
2008: May 31 - Loses 100m world record when countryman Usain Bolt runs 9.72secs in New York.
August - Wins gold and helps set new world record at the Beijing Olympics as part of Jamaica's 4x100m team. Finishes fifth in 100m individual final.
September 8 - Sets 100m personal-best with 9.72secs in Lausanne, just outside the new world mark of 9.69secs set by Bolt at Beijing Olympics.
2009: August - Wins 4x100m gold at World Championships in Berlin and earns bronze in 100m behind Bolt and Tyson Gay. Bolt running a new world record of 9.58secs in taking the victory.
2012: August 5 - Finishes eighth in final of London Olympics 100m after injuring groin in race.
2013: July 14 - Powell's management company confirm he had failed a drugs test.