The WBC has confirmed next Saturday's heavyweight clash between Deontay Wilder and Alexander Povetkin has been postponed.
American Wilder (36-0-KO35) was due to head to Moscow on Monday for the final preparations for the fourth defence of his belt but his Russian opponent failed a drugs test.
Povetkin's sample tested positive for meldonium, a substance that was added to the World Anti-Doping Authority (WADA) banned list last year and caused tennis star Maria Sharapova to be banned.
Promoter Andrei Ryabinsky confirmed Povetkin had been taking meldonium before it was banned and that only "leftover traces of meldonium at a very low concentration" were found in a blood sample given last month.
But the WBC has decided the bout is off, pending an investigation, with no back-up 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."