Former world champion Parker produced a positive sample during a doping test carried out the very day he fought in 11th-round defeat against English fighter Fabio Wardley.
The British boxer and Parker were part of an intense encounter in London last October to establish the next next in line for the WBO against undisputed world heavyweight champion Ukrainian champion Usyk.
Parker was previously the leading contender to face the champion after putting together a notable six wins in a row, however risked his contender position on the line during the bout and endured an 11th-round stoppage.
A routine test administered to Joseph Parker through Vada the day he fought showed an adverse result and leaves Parker in a difficult position to clear his name.
The boxer's representatives released a communique stating: "The agency notified everyone involved about Parker tested positive following a drug screening conducted on October 25th. While the matter is investigated, no more comments will be made currently."
The New Zealand boxer was crowned titleholder six years ago until he was defeated for his championship belt through a points decision versus Anthony Joshua at the Millennium Stadium in 2018.
The fighter became a frequent visitor in the UK from that point, competing in eight bouts overall in Britain and moving to Ireland for preparation alongside respected coach former champion Lee in recent years.
Under the tutelage of Lee, Joseph Parker bounced back from the 2022 setback versus Joe Joyce to achieve a string of victories positioning him at the front of the line to challenge the champion before he lost to Wardley.
After the boxer produced a positive test when he fought Wardley, allegedly for cocaine, a probe into the findings has commenced.
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