BREAKING NEWS: Anthony Joshua’s heavyweight championship defence against Jarrell Miller is in doubt after a positive drug test

Anthony Joshua’s American debut is in jeopardy after his challenger Jarrell “Big Baby” Miller allegedly tested positive for a banned substance.

ESPN first reported Miller failed a doping test for banned substance GW1516, citing three sources with knowledge of the results.

The heavyweights were scheduled to fight on June 1 at New York’s Madison Square Garden, with Joshua (22-0) defending his WBA, WBO and IBF heavyweight titles.

Ahead of the fight, the pre-sale tickets broke an MSG record — and it took only two hours to become the highest grossing pre-sale in the history of the arena.

“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 night,” Rafael tweeted.

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

GW1516, also known as cardarine and endurobol, is on the World Anti-Doping Agency’s banned list and classified as a metabolic modulator. An athlete might use it to assist in fat loss or use it as an endurance booster, per ESPN.

Joshua’s promoter Eddie Hearn confirmed the report but said his fighter’s “preparation continues” for the big date.

Miller, who has a professional record of 23 wins and one draw from 24 fights, is a former kickboxer who was suspended for nine months and fined $2500 in 2014 when he tested positive for the substance dimethylamylamine.

The 30-year-old, with no evidence, had accused Joshua of using performance-enhancing drugs earlier this year.

“I study every opponent,” said Miller, at a press conference in February. “My dad was a bodybuilder growing up; you can see it. I look at someone and I say, he’s on steroids. Anybody who comes out of the amateurs at 220lbs and then is 240lbs in a year, there’s something wrong with that picture.

“There’s no way you’re going to put on 20lbs of muscle within a year without taking some kind of supplement. It’s impossible. I know bodybuilders train all year round, non-stop, and don’t put on that amount of muscle.

“Now, if you look at his stature, while he’s staying at this higher level, his physique has kinda shrunk.

“We’ve both got contracts in place where we’re protected and the proper testing is going to be taking place, but it’s written clear as day.”

Joshua laughed off the accusation at the time. “Jarrell has been banned for drugs,” Joshua said, “so maybe that’s why he’s saying it.”