Ryan Garcia has protested his innocence after WBC president Mauricio Sulaiman said the American boxer had returned "adverse findings" in a drugs test before last month's shock win over Devin Haney.
Garcia floored Haney three times en route to an upset majority decision victory on April 20, although he had already missed out on a chance to claim his compatriot's WBC light-welterweight crown after missing the required weight for the fight.
Now ESPN report that the the Voluntary Anti-Doping Association (VADA) has informed all parties that Garcia failed tests for the banned substance ostarine, which can be used to aid muscle mass and stamina. The samples were allegedly collected the day before and the day of his win over Haney.
Sulaiman wrote on X: "The WBC has received just now notification from Vada-Testing about Ryan GarcIa adverse findings on tests taken with connection to his fight against Haney.
"The due process begins immediately as per the WBC Clean Boxing Program Protocol. More information to follow."
Garcia, who improved his professional record to 25 wins from 26 fights with his win over Haney, took to social media to deny taking a performance enhancing substance. He has ten days to appeal the finding.
In a video posted on his X account, Garcia said: "Everybody knows that I don't cheat. I've never taken a steroid - I don't even know where to get steroids at the end of the day. I barely take supplements."
Garcia, 25, then posted a further string of tweets appearing to make light of the situation, which was criticised by Haney.
In a statement, Haney, 25, said: "It's unfortunate Ryan cheated and disrespected both the fans and the sport of boxing by fighting dirty and breaking positive not once, but twice.
"Ryan owes the fans an apology, and by his recent tweet he still thinks this is a joke. We put our lives on the line to entertain people for a living. You don't play boxing.
"This puts the fight in a completely different light. Despite the disadvantage, I still fought on my shield and got back up. People die in this sport. This isn't a joking matter."
The PA news agency has contacted VADA for comment.