Jarrell Miller could miss out on a shot at Anthony Joshua's world heavyweight titles after the American produced an "adverse finding" in a drug-testing sample.
Miller, who is undefeated in his 24 professional fights, was due to take on the Briton at Madison Square Garden for the WBA, IBF, IBO and WBO titles on June 1.
However, both camps have confirmed the Voluntary Anti-Doping Association (VADA) had informed them that Miller had returned the results from a test on March 20.
Miller is expected to face a hearing before the New York State Athletic Commission to decide whether he will be allowed to fight.
Miller previously served a nine-month ban from the California State Athletic Commission after testing positive for methylhexaneamine when kickboxing in 2014.
During their promotional tour for the fight, Miller revealed that both fighters had agreed to between 10 and 12 weeks of drug testing in the build-up to the bout.
Joshua, during the London leg of the tour, said: "I am on (drug testing scheme) ADAMS whereabouts, they have to know where I am for an hour every day and could walk in anytime.
"They can turn up any time during the day and that is a lifetime commitment. I have been on that since 2012 and I pay £40,000-a-fight to be tested by VADA two times a week."
WBA "regular" champion Manuel Charr (31-4, 17 KOs) is one of the frontrunners to replace Miller on the date.
“Internal negotiations are taking place regarding the fight in June Mahmoud Charr against Anthony Joshua,” Charr stated.
“I am very happy about this task and the challenge. I’m ready to go to fight against Joshua and bring home the victory.
“As the No.1 WBA and reigning world champion, I’m the most worthy opponent for Anthony Joshua. I am the suitable and official challenger. For Joshua’s honour, it’s important to fight against the No.1 WBA rankings in the world.”