Australian media reports say former WBF heavyweight champion Francois Botha tested positive to the banned stimulant Phentermine before his bout with rugby player Sonny Bill Williams on Friday.
The reports said the banned substance was found in a urine sample taken on Tuesday from the 44-year-old South African, who has fought Mike Tyson, Evander Holyfield, Michael Moorer and Lennox Lewis during a 20-year-professional career.
The test also found traces of Benzodiazepine, a valium-like product, in Botha's system. Results will be sent to the Panama headquarters of the WBA, which sanctioned Friday's bout.
Botha lost a unanimous but contentious points decision to Williams.