The New Zealand heavyweight tested positive as part of a doping test carried out the same day he fought in 11th-round stoppage against English fighter Fabio Wardley.
Wardley along with Parker were part of a thrilling bout held in London on 25 October to establish the next next in line for the WBO against undisputed world heavyweight champion Oleksandr Usyk.
The fighter had positioned himself as the leading contender to challenge Usyk after putting together a notable six consecutive victories, but risked his shot at the title in jeopardy in London and suffered an 11th-round defeat.
An ordinary examination administered to Parker by Vada on the day of the bout showed an adverse result and now leaves the 36-year-old with a battle to clear his name.
His promotion team released a communique declaring: "The anti-doping body informed all relevant parties about Parker tested positive after an anti-doping test conducted on October 25th. As the investigation continues, no additional comment are planned at this time."
Joseph Parker earned the title of world champion in 2016 prior to he was defeated for his WBO belt by unanimous decision against British star Joshua in Cardiff two years later.
Parker remained an active participant across the United Kingdom from that point, fighting eight times altogether on UK territory and relocating to Ireland for fight camps alongside highly regarded trainer former champion Lee over the past few years.
Guided by Lee, Parker recovered from his 2022 loss to British heavyweight Joyce to achieve a string of victories positioning him at the front of the line to face the champion until he lost to the British fighter.
Following Parker returned a positive test during the Wardley bout, reportedly involving cocaine, an investigation into the findings has been started.
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