Saul 'Canelo' Alvarez has tested positive for trace levels of the banned drug clenbuterol, with his promoters blaming contaminated meat.
Golden Boy Promotions confirmed via a news release that Alvarez provided the positive sample as part of the voluntary testing programme he requested ahead of his rematch with Gennady Golovkin in Las Vegas on May 5.
The statement added that a "meat contamination that has impacted dozens of athletes in Mexico over the last years" was the reason for the positive test, and that the Nevada State Athletic Commission and Golovkin's promoter Tom Loeffler have been notified.
The fight is still set to go ahead and Alvarez, who will move his training camp from Mexico to the United States, said: "I am an athlete who respects the sport and this surprises me and bothers me because it had never happened to me.
"I will submit to all the tests that require me to clarify this embarrassing situation and I trust that at the end the truth will prevail."
Golden Boy's statement added that Daniel Eichner, director of Sports Medicine Research and Testing Laboratory who conducted the tests, said via a letter: "These values are all within the range of what is expected from meat contamination."
The first bout between Alvarez and fearsome Kazakh Golovkin ended in a contentious draw.