Jarrell Miller has denied he is a drug cheat after testing positive for a banned substance.
Miller is due to challenge world heavyweight champion Anthony Joshua in New York on June 1 - but that is now in serious doubt.
The Voluntary Anti-Doping Association informed Joshua's camp about an adverse finding in a sample collected last month.
Promoter Eddie Hearn tweeted: "We have been informed by VADA that there has been an adverse finding in Jarrell Miller's sample collected on March 20th, 2019.
Jarrell Miller fails drugs test ahead of Anthony Joshua fight
"We are working with all relevant parties and will update with more details soon. AJ's preparation continues for June 1st at MSG."
Miller has yet to speak out about test but Sarah Fina, vice president of operations for his promotional company, tweeted: "Jarrell obviously denies the allegations... in no way shape or form do I condone cheating, but for now I stand by our fighter."
Joshua and Miller were due to fight for the WBA, IBF and WBO titles at Madison Square Garden.
Although Joshua's preparations will continue the VADA news leaves the Miller fight in doubt and a replacement fighter may need to be found for what would be Joshua's USA debut.
