Anthony Joshua's defence of his world titles is in serious doubt after Jarrell Miller failed a drug test.
Joshua is due to put his IBF, WBA and WBO heavyweight belts on the line against Miller at Madison Square Garden in New York on June 1.
But the American, who has repeatedly accused Joshua of cheating, could now face a ban after testing positive for a banned substance.
Joshua's promoter Eddie Hearn said: "We have been informed by VADA that there has been an adverse finding in Jarrell Miller’s sample collected on March 20.
"We are working with all relevant parties and will update with more details soon. AJ's preparation continues for June 1 at MSG."
Miller tested positive for GW1516, a substance which increases aerobic power and endurance.
It was added to the World Anti-Doping Agency's banned list in 2009 and reportedly costs up to $1000 for 10g on the black market.
In 2012, four Costa Rican cyclists tested positive for the substance and they were followed by a Russian the following year; all were banned.
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Joshua is still expected to fight on June 1 with Hearn scrambling to find an opponent.
The Brit was originally expected to fight Dillian Whyte at Wembley this month but a deal could not be reached.
He also failed to agree terms with fellow world champion Deontay Wilder who takes on Dominic Breazeale on May 18.
