Real Madrid captain Sergio Ramos has denied committing a doping offence after the 2017 Champions League final.
The Spanish defender allegedly tested positive for anti-inflammatory drug dexamethasone after his side's 4-1 win over Juventus in Cardiff, according to Der Spiegel.
The substance is only banned if testers are not informed it has been used, with the German magazine reporting that Ramos had declared a different substance.
An "administrative error" was given as the reason for the situation with a club doctor blamed.
"I am vehemently opposed to doping," said Ramos.
"I have never participated, nor will I ever participate in, nor have I consented to, nor will I ever consent to, any form of doping.
"Over the course of my long sporting career, I have submitted to, and passed satisfactorily, an infinite number of anti-doping tests."
Ramos said that he was considering legal action against anyone who "undermined his reputation" or "violated his rights".
European football body UEFA reportedly accepted an apology from Ramos and ruled that an administration error had most likely occurred.
They have denied any accusation of a "cover-up".
"UEFA strongly and categorically refutes unfounded allegations it has covered up positive doping results," a statement said.
"All UEFA doping control cases are conducted in full compliance with the World Anti-Doping Agency Code."
In a letter seen by Der Spiegel, UEFA reportedly said to Ramos: "In the future, we ask you and your team doctor to be utmost cautious when completing the doping control form and more precisely the declaration of medication."
A Real Madrid statement said: "Ramos has never breached the anti-doping control regulations.
"UEFA requested timely information and closed the matter immediately, as is usual in these cases, after verification by the experts themselves."