French tennis player Richard Gasquet tested positive for cocaine at a tournament in Miami last month, the world number 23 said on Sunday.
"The B sample test from March 2009, during the Miami tournament, in which I did not take part, confirms the positive results from the A sample," he said in a statement read out on French radio RTL.
Gasquet said he had a hair sample tested by an independent lab "which showed no trace of cocaine".
"Given the complexity of the case, I am gathering the evidence of my innocence and will later set a date to make further comments," the 22-year-old added.
Gasquet had pulled out of the Sony Ericsson Open without playing a match, citing a shoulder injury.
If Gasquet fails to clear his name, he could face a two-year suspension from the sport.
"My first reaction is one of sadness. I know the kid's character well and I am very surprised," French tennis federation (FFT) president Jean Gachassin told French radio RMC.
"I want to talk to Richard myself to know exactly what happened.
"He has to explain himself, for French tennis, for the young players. I want explanations. Then, I will take action."
International Tennis Federation (ITF) president Francesco Ricci Bitti, who is in Montreal attending the World Anti-Doping Agency's executive board meeting, said: "I don't know the details of this case. But it is always sad when something like this happens. But we are not prosecutors."
French veteran Fabrice Santoro, who trains under the same Team Lagardere structure as Gasquet, told French radio Europe 1: "I have known Richard since he was nine so I am very surprised.
"I know his lifestyle and I am extremely surprised because it does not fit the character. I spoke to him over the phone, he is sad and very shocked."
Gasquet is not the first tennis player to fail a test for the recreational drug.
Former women's world number one Martina Hingis tested positive for the banned substance in 2007 and promptly announced her retirement from the sport, saying she had "no desire to spend the next several years of my life reduced to fighting against the doping officials."
Former men's world number one Mats Wilander of Sweden was suspended for three months for cocaine use along with doubles partner Karel Novacek at the 1995 French Open.
Gasquet, who was regarded as the top French prospect when he started his professional career in 2002, has failed to live up to expectations.
He ended his debut season as the youngest player to finish in the Top 200 after also becoming the youngest to qualify for an ATP Masters at Monte Carlo.
Gasquet has always been scrutinised by French media and was on the cover of French Tennis Magazine at the age of nine.
In February 2008, he launched the Richard Gasquet Foundation, which helps adolescents struggling to find their place in society and suffering from a lack of confidence.