Heavyweight contender Jarrell "Big Baby" Miller has tested positive for the banned substance GW1516 ahead of his scheduled challenge of unified world titleholder Anthony Joshua, three sources with knowledge of the results told ESPN on Tuesday.

The fight is scheduled to take place on June 1 (DAZN) at Madison Square Garden in New York and to serve as the American debut for England's Joshua, but it is now in jeopardy because of Miller's positive test.

The test results, which came to light on Tuesday, were from a random Voluntary Anti-Doping Association-conducted urine test Miller submitted to on March 20.

Miller (23-0-1, 20 KOs), 30, had accused Joshua of using PEDs during their media tour to announce the fight in February.

"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 after he was informed of the positive test result. "In the meantime, Jarrell continues to train for his June 1 fight against Anthony Joshua."

Matchroom Boxing promoter Eddie Hearn, who represents Joshua, also acknowledged the positive test.

"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 in a statement. "We are working with all relevant parties and will update with more details soon. AJ's preparation continues for June 1st at MSG."

If Miller is dropped from the fight, it will cost him millions. He is due to earn a career-high purse of $4.875 million, not to mention additional money he stands to earn from a percentage of the profits from the fight on Sky Sports Box Office pay-per-view in the United Kingdom. Miller's deal with DAZN calls for two comeback fights worth $1.5 million apiece in the event that he loses to Joshua, which would disappear if there is no fight.

The biggest purse of Miller's career was a little over $500,000 for his fourth-round knockout of Bogdan Dinu on Nov. 17.

GW1516, also known as Cardarine and Endurobol, is on the World Anti-Doping Agency's banned list and is classified as a metabolic modulator. An athlete might use it to assist in fat loss or use it as an endurance booster. WADA has warned athletes of health risks connected to the drug, which in early trials caused cancer in laboratory mice.

Miller is likely to face a hearing on the matter before the New York State Athletic Commission, which probably will decline to license him for the fight if it handles this as it has similar recent situations. On Dec. 22, for example, Willie Monroe Jr. was due to challenge interim middleweight world titlist Jermall Charlo at Barclays Center in New York, but when Monroe tested positive for elevated levels of testosterone, the commission would not license him, and Charlo faced late replacement Matt Korobov instead.

Whomever Joshua (22-0, 21 KOs), 29, faces, he will be making his seventh title defense and fighting outside the United Kingdom for the first time. The 6-foot-4, 300-pound Miller got the matchup with Joshua thanks to his ability to sell a fight with his mouth and because he defeated a series of former world title challengers the past two years to earn his title shot.