American Jarrell Miller has returned an "adverse finding" in a voluntary drug test ahead of his world heavyweight title fight with Anthony Joshua.
The finding could disrupt Miller's upcoming bout with world champion Joshua, scheduled for June 2 (NZ time) in New York.
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Joshua's boxing promoter, Eddie Hearn, said he had been informed of Miller's positive test by the Voluntary Anti-Doping Association (VADA), which works with athletes in boxing and mixed martial arts.
"We have been informed by VADA that there has been an adverse finding in Jarrell Miller's sample collected on March 20th, 2019," Hearn said on his Twitter account.
"We are working with all relevant parties and will update with more details soon. AJ’s preparation continues for June 1st."
ESPN reported that Miller had tested positive for the banned substance GW1516, which boosts endurance and helps athletes burn fat, citing three sources familiar with the matter.
Miller's co-promoter, Dmitriy Salita, said the 30-year-old boxer would continue to train, while his team sought more information.
"We are in the process of obtaining further information about VADA’s finding and will have more to say soon on this developing situation," Salita told ESPN.
"In the meantime, Jarrell continues to train for his June 1 fight against Anthony Joshua."
VADA did not immediately respond to a Reuters e-mail requesting comment.
In February, Miller accused Joshua of using Therapeutic Use Exemptions (TUEs) to take testosterone that helped him build muscle, a charge that Joshua’s camp denied.