The New Zealand heavyweight produced a positive sample during a drugs test administered on the day of his 11th-round loss to British heavyweight Ipswich-born Wardley.
Fabio Wardley and the New Zealander participated in an intense encounter in London last October to decide who would become mandatory WBO contender facing reigning heavyweight king Oleksandr Usyk.
Parker had positioned himself as the primary candidate to meet the champion after compiling an impressive six consecutive victories, but placed his contender position on the line during the bout and suffered a stoppage in round eleven.
An ordinary examination given to Parker through Vada the day he fought came back positive and now leaves the 36-year-old with a battle to demonstrate his cleanliness.
The boxer's representatives published an announcement stating: "Vada informed everyone involved that Joseph Parker had a positive result after a drug screening conducted on October 25th. During the ongoing inquiry, no more comments will be made at this time."
Parker was crowned titleholder back in 2016 until he was defeated for his WBO belt via unanimous decision against British star Joshua in Wales 24 months later.
The fighter has been an active participant in the UK since then, having eight matches in total in Britain and relocating to Dublin for training camps alongside esteemed mentor Andy Lee during this period.
Under the tutelage of Lee, Parker bounced back from his defeat in 2022 against Joe Joyce to secure a string of victories positioning him front of the queue to challenge Oleksandr Usyk prior to he suffered defeat against Wardley.
Following Parker returned an adverse finding during the Wardley bout, reportedly involving cocaine, a probe about the test outcome is underway.
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