The New Zealand heavyweight tested positive during a doping test carried out the very day he faced 11th-round stoppage to English fighter Fabio Wardley.
Fabio Wardley together with Joseph Parker were part of a thrilling encounter in London on 25 October to establish who would become next in line for the WBO to reigning heavyweight king Ukrainian champion Usyk.
The fighter had been the leading contender to meet Usyk after stringing together a remarkable six wins in a row, yet placed his contender position in jeopardy in the UK and suffered an 11th-round defeat.
A routine test given to the boxer through the anti-doping organization the day he fought showed an adverse result which leaves Parker in a difficult position to clear his name.
The boxer's representatives released a communique which read: "The anti-doping body informed all required parties about Parker returned an adverse finding following a drug screening administered on the 25th October. During the ongoing inquiry, no further statements will be made for now."
Joseph Parker was crowned titleholder six years ago until he surrendered the WBO title through a points decision to Anthony Joshua in Wales 24 months later.
Parker remained an active participant in the UK from that point, fighting eight times overall on UK territory and relocating to the Irish capital for fight camps with esteemed mentor Andy Lee during this period.
Coached by Andy Lee, the boxer bounced back from his defeat in 2022 versus British heavyweight Joyce to secure a string of victories positioning him as the top contender to challenge Oleksandr Usyk prior to he lost against Wardley.
Subsequent to the boxer produced a positive test during the Wardley bout, reportedly for cocaine, a probe about the test outcome has been started.
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