Russian boxer Alexander Povetkin has been suspended indefinitely by the World Boxing Council and fined $250,000 for doping, the WBC said.
Povetkin tested positive for the banned substance meldonium last year, just before he was due to face WBC heavyweight champion Deontay Wilder in a May 21 title fight in Moscow.
The WBC initially cleared him last August, saying there was insufficient evidence he had used the performance-enhancing drug after it was banned by the World Anti-Doping Agency on January 1, 2016.
However, Povetkin again tested positive for a banned substance in December -- this time ostarine, an anabolic steroid-like drug.
That, together with the earlier meldonium test, caused the WBC to slap him with an indefinite ban and a hefty fine, in a decision released Thursday night.
The boxer will be allowed to request his suspension be lifted after one year, the WBC said in a statement.
Povetkin, 37, won the gold medal in the super-heavyweight category at the 2004 Olympics in Athens.
Dozens of athletes, mainly Russian, tested positive last year for meldonium.
The drug, developed in the former Soviet Union in the 1970s, has been propelled into the spotlight since Russian tennis star Maria Sharapova tested positive for it at the Australian Open in 2016.