Middlesbrough's Abel Xavier insists he never intended to "dope himself" and will fight to prove his innocence after a positive drugs test. Boro revealed on Monday that the 32-year-old had failed a test following the club's Uefa Cup match against Xanthi on 29 September.
In a statement released late on Tuesday, Xavier said: "I am convinced there is a harmless explanation.
"In order to furnish this proof, I am undergoing a number of tests."
The Portuguese defender did not reveal the name of the substance found in his system nor whether it was a recreational or performance-enhancing drug.
He has been suspended from all competitions while an investigation takes place.
Uefa guidelines recommend a ban of 12 months for a first doping offence.
His statement read: "As agreed with my employer, on Monday evening Middlesbrough informed the press that in the anti-doping control I had to undergo after the above-mentioned Uefa Cup match, I tested positive.
 |  I have never had the intention to dope myself |
"I hereby confirm this fact.
"This positive test result was found in the 'A' sample. In the course of next week, the 'B' sample will be analysed.
"I am convinced that there is a reasonable and entirely harmless explanation for such positive finding, should it be confirmed by the analysis of the 'B' sample.
"In order to furnish this proof, I am currently undergoing a number of tests in which certain of my body fluids and tissues are analysed."
The former Liverpool and Everton player said he was determined to provide "scientific and factual proof" that he did not deliberately take a banned substance.
"I have never had the intention to dope myself," the statement continued.
Xavier requested he be given "the time to conduct the above-mentioned research and analyses under the best possible conditions".
He added: "I am therefore not in a position to communicate with the press in more detail for the time being.
"However, I will certainly do so as soon as possible in order to prove, as I hope, my innocence, and my good faith."
John Scott, UK Sport's director of drug-free sport, says he was surprised by news of Xavier's positive test.
"It did surprise me," he told BBC Radio Four.
"For a long time, there has been information out there that all athletes should be extremely wary of a substance.
"Anyone who chooses to use supplements is dicing with an extremely dangerous product where drug testing is concerned."
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Report: BBC Sport's Gordon Farquhar