Cuba's Luis Ortiz has failed a drug test only weeks before he was due to challenge for the World Boxing Council (WBC) heavyweight title.
WBC President Mauricio Sulaiman revealted they were notified of the positive drug test by the Voluntary Anti-Doping Association (VADA), part of the governing body's clean boxing programme.
Ortiz was due to box WBC heavyweight champion Deontay Wilder on November 4 in a major event at the Barclays Centre in Brooklyn in New York City.
While Ortiz can appeal, this is not the first time he has failed a drug test.
He was World Boxing Association interim heavyweight champion from 2015 to 2016 but his first reign was nullified via no contest when he was stripped of the title when it was revealed he had tested positive for prohibited substance in 2014.
The 38-year-old, nicknamed "The Real King Kong", has won 27 of his 29 professional fights, 23 of them by knock-out.
Before turning professional in 2010, Ortiz won 343 of his 326 amateur fights.
Among the titles he won was the World Cup in Moscow and the Panamerican Championships in 2005.
The winner of Wilder-Ortiz would have been most likely to fight Anthony Joshua, in one of the most lucrative showdowns in the sport, in 2018.
Wilder, the Olympic bronze medallist at Beijing 2008, has lost out on big fights before due to opponents failing drug tests, notably a significant showdown with Alexander Povetkin in Russia in 2016.
Ahead of a fight in February this year, his original opponent Andrzej Wawrzyk also tested positive for a banned substance.
At least, there are substitutes on the November 4 bill, with former world champion Bermane Stiverne and former world title challenger Dominic Breazeale due to box each other.