Lucas Browne has been stripped of his WBA heavyweight title after his 'B' sample tested positive for a banned substance.
Browne returned a positive 'A' sample for clenbuterol after stopping Ruslan Chagaev on March 5 to become Australia's first heavyweight world champion.
The 37-year-old, who signed up to a voluntary anti-doping programme in the run-up to the fight, has repeatedly protested his innocence and insists he is a clean fighter.
But a second positive sample has triggered an automatic six-month ban from the WBA, who say they will reconsider his status when his suspension is up.
Browne tweeted in response: "I'm leaving everything to my lawyer, Mr Leon Margules. I thank everyone for their support, especially the people and fans of Australia."
A statement on the WBA website said Chagaev now has 120 days to negotiate a fight with Fres Oquendo, who is mandatory challenger to the winner of the upcoming meeting between interim champion Luis Ortiz and Alexander Ustinov.