Heavyweight contender Jarrell "Big Baby" Miller tested positive for a banned substance, jeopardizing his planned challenge of unified world titleholder Anthony Joshua, ESPN's Dan Rafael reported.
The Miller-Joshua bout is scheduled for June 1 at Madison Square Garden in New York.
Rafael said a random urine test conducted March 20 by the Voluntary Anti-Doping Association was positive for the substance GW1516, which is on the World Anti-Doping Agency's banned list. It can be used to help to lose fat or to boost endurance.
In a twist, Miller (23-0-1, 20 KOs) had accused Joshua of using performance-enhancing drugs earlier this year.
"We are in the process of obtaining further information about VADA's finding and will have more to say soon on this developing situation," Dmitriy Salita, Miller's co-promoter with Greg Cohen, told ESPN. "In the meantime, Jarrell continues to train for his June 1 fight against Anthony Joshua."
Matchroom Boxing promoter Eddie Hearn, who represents Joshua, said his fighter also is continuing to train for the scheduled bout.
The biggest purse to date of Miller's career - about $500,000 - came last November when he knocked out Bogdan Dinu. If Miller is removed from the June fight card because of the positive test, he will lose a purse of $4.875 million, plus pay-per-view percentages and two comeback fights at $1.5 million each if he were to lose.
The Madison Square Garden fight would represent the U.S. debut for Joshua (22-0, 21 KOs), a 29-year-old from England.
--Field Level Media