Queensberry Promotions have confirmed Joseph Parker failed a drug test on Oct. 25, the day of his fight against Fabio Wardley in London.
Parker suffered a Round 11 stoppage defeat at London's O2 Arena with Wardley, losing his status as WBO mandatory challenger for undisputed heavyweight champion Oleksandr Usyk.
"The Voluntary Anti-Doping Association [VADA] last night informed all required parties that Joseph Parker returned an adverse finding following an anti-doping test conducted on 25 October in relation to his bout with Fabio Wardley," Queensberry said in a statement on Friday.
"While the matter is investigated further, no additional comment will be made at this time."
The news was first reported by The Sun, who said Parker tested positive for cocaine, although his has not been confirmed.
Parker had positioned himself for a potential fight against Usyk after six-straight wins including victories over Deontay Wilder, Zhilei Zhang and Martin Bakole.
However, he could face a ban depending on the outcome of the investigation.
ESPN has contacted Parker's team and VADA for comment.