Heavyweight champion Deontay Wilder's title defense against Alexander Povetkin has been postponed by the WBC after the Russian challenger failed a drug test.
The WBC says the May 21 fight in Moscow is off, pending an investigation, with no backup date.
"Keeping the priority of safety and also the principle of justice, the WBC will continue the investigation into the case," the WBC said in a statement.
"Consequently, the event scheduled for May 21 in Moscow is hereby officially postponed."
It added that "the WBC will be releasing more information in the coming days regarding the final ruling on the matter."
The postponement does not affect a cruiserweight unification bout between WBA super champion Denis Lebedev and IBF champion Victor Emilio Ramirez, which had been on the undercard of the Wilder-Povetkin fight. That bout will go ahead as planned, event spokeswoman Camilla Ahrorbekova told the AP.
Povetkin's promoter has said the substance found in an April test was meldonium, the stamina booster for which tennis star Maria Sharapova and dozens of other athletes in ex-Soviet nations have tested positive since it was banned from Jan. 1.
Wilder, an unbeaten American (36-0), had been due to make his fourth defense of the belt he won against Bermane Stiverne in January last year. For Povetkin (30-1), it is his first world title challenge since losing a unanimous decision to Wladimir Klitschko in 2013.
Wilder had made allegations against Povetkin in an interview last year with USA Today. Wilder said he believed the Russian to be "on some kind of steroids" but said he was still willing to fight him.
Povetkin's promoter Andrei Ryabinsky told Russia's Tass news agency on Friday that his fighter had tested positive in April, but that only "leftover traces of meldonium at a very low concentration were found." These traces, Ryabinsky said, were the result of Povetkin taking meldonium in September last year before it was banned.
Ryabinsky said that he accepted the WBC's ruling and hopes to restage the fight at a later date once the meldonium is out of Povetkin's system. "It'll happen but on another date," he said in a statement.
Athletes in several sports have avoided suspensions for meldonium positive tests under new World Anti-Doping Agency guidance issued in April - if the concentration of the heart drug is low enough to indicate they did not take it after Jan. 1.
It is not clear whether such guidance will apply in the case of Povetkin. The WBC works with the Las Vegas-based Voluntary Anti-Doping Association, which often follows WADA's example but is not under its supervision.
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