Anthony Joshua’s next opponent Jarrel Miller has returned an "adverse finding" in a drug-testing sample.
Promoter Eddie Hearn made the announcement, claiming he had been informed by the Voluntary Anti-Doping Association (Vada) about the finding from a sample collected on 20 March.
Briton Joshua is scheduled to defend his IBF, WBA and WBO heavyweight titles against the undefeated American, who has never fought for a world title.
Hearn said Joshua's "preparations continue" for his US debut.
Miller's promoter, Dmitriy Salita, said: "We are in the process of obtaining further information about Vada's finding and will have more to say soon on this developing situation. In the meantime Jarrell continues to train for his 1 June fight against Anthony Joshua."
Joshua's manager Freddie Cunningham said they "will 100% be looking at replacement options" if Miller is not allowed to fight.
Miller, 30, is due to fight Joshua, 29, at Madison Square Garden in New York on 1 June.
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