Former world champion Parker produced a positive sample as part of a anti-doping examination carried out the very day of his 11th-round stoppage versus British heavyweight Fabio Wardley.
Wardley together with Parker engaged in a pulsating encounter in London last October to determine the identity of the WBO mandatory challenger to undisputed world heavyweight champion Ukrainian champion Usyk.
Parker had positioned himself as the leading contender to challenge Oleksandr Usyk following compiling an impressive six wins in a row, yet risked his shot at the title on the line in the UK and experienced an 11th-round defeat.
A standard screening administered to the boxer through the anti-doping organization on the day of the bout came back positive and now leaves Parker with a battle to demonstrate his cleanliness.
His promotion team released an announcement which read: "The agency notified all relevant parties regarding Joseph Parker had a positive result subsequent to an anti-doping test carried out on October 25th. As the investigation continues, no further statements will be made currently."
Parker was crowned heavyweight champion six years ago prior to he was defeated for the WBO title through a points decision to Anthony Joshua at the Millennium Stadium in 2018.
The fighter remained a frequent visitor across the United Kingdom after that, competing in eight bouts in total on UK territory and relocating to the Irish capital for fight camps under esteemed mentor Andy Lee during this period.
Guided by Lee, Joseph Parker overcame his 2022 loss against British heavyweight Joyce to achieve multiple wins that placed him at the front of the line to meet the champion before he suffered defeat to the British fighter.
Following the boxer produced a failed test on the day of the Wardley fight, reportedly involving cocaine, a probe into the findings has been started.
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