Jaime Munguia insisted on Thursday he has "always been a clean athlete" as it emerged the Mexican boxer tested positive for a banned substance after his victory over France's Bruno Surace on 4 May.
The Ring Magazine reported on its website on Thursday that the Voluntary Anti-Doping Association (VADA) had "informed all relevant parties" that the "A" sample provided by Munguia was positive for exogenous origin of testosterone metabolites.
If the "B" sample returns a similar adverse finding Munguia faces suspension and the the change of his victory to a no-contest.
"Several experts have explained that there are multiple ways contamination can occur, and I am fully willing to undergo any retroactive, current, or future testing to demonstrate that I have always been a clean athlete," Munguia said in a statement posted on Instagram.
The 28-year-old, who owns a record of 45-2 with 35 knockouts, said he has undergone numerous anti-doping tests in his career and never tested positive.
"I was tested twice during this training camp, and both results came back negative, which is why receiving this notification of an adverse finding has been a complete surprise to me," said Munguia, who added he would not comment further until the "B" sample results are known.
Munguia earned a unanimous decision victory over Surace on May 4 in Riyadh, on the undercard of Canelo Alvarez's super middleweight title unification triumph over William Scull.
Munguia kept Surace on the ropes for much of the match as he avenged a knockout loss to the French fighter in December.
Surace told The Ring that the result of the rematch should be "overturned immediately".
"I went to Mexico and knocked out Mungiua fair and square in December and I was gracious enough to give him a rematch in May," he said.
"He knew he could not beat me n a level playing field. I trust that this result will be overturned immediately".