Jarrell Miller has recorded an "adverse finding" in a drug test ahead of his world heavyweight title fight with Anthony Joshua.
The American is due to fight IBF, WBA, and WBO champion Joshua on June 1 at Madison Square Garden in New York, live on Sky Sports Box Office.
Hearn said on Twitter of the Voluntary Anti-Doping Association (VADA) test: "We have been informed by VADA that there has been an adverse finding in Jarrell Miller's sample collected on March 20, 2019.
"We are working with all relevant parties and will update with more details soon.
"AJ's preparation continues for June 1 at MSG."
Miller's co-promoter Dmitry Salita confirmed the news, saying his boxer would continue to train as they sought more information.
"We are in the process of obtaining further information about VADA's finding and will have more to say soon on this developing situation," Salita Promotions said on Twitter.
"In the meantime, Jarrell continues to train for his June 1 fight against Anthony Joshua."