Tyson Fury is reported to have tested positive for cocaine and may now be stripped of his world titles.
Fury reportedly failed a Voluntary Anti-Doping Association (VADA) conducted test last week, with cocaine found in a random urine sample. The results of VADA's test are not yet complete however.
Fury pulled out of his title defence fight against Wladimir Klitschko, scheduled for October 29, with his team saying he had been declared 'medically unfit' on September 23.
They said: "Tyson has, this week, been declared medically unfit to fight.
"Medical specialists have advised that the condition is too severe to allow him to participate in the rematch and that he will require treatment before going back into the ring."
However, according to reports from ESPN, Fury had tested positive for the drug the day before on September 22.
A source close to the planned fight told Press Association Sport he had been informed "traces" of cocaine were found in Fury's system. The 28-year-old fighter's camp would not confirm that.
A spokesman for his promoter Hennessy Sports declined to discuss the claims, saying the Fury camp would not be making a statement.
Fury caused a major upset when he beat veteran Ukrainian Klitschko last November in Dusseldorf.
One attempt at a rematch of the world title fight had already been abandoned because of an ankle injury sustained by Fury.