New Zealand's former heavyweight world champion Joseph Parker failed a dope test on the day he fought Britain's Fabio Wardley last month, Frank Warren's Queensberry Promotions confirmed on Friday.
British media had reported the 33-year-old, whose whereabouts were unclear, tested positive for traces of cocaine and could face a lengthy ban from the sport.
"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 the 25th October in relation to his bout with Fabio Wardley," Queensberry said in a statement.
"While the matter is investigated further, no additional comment will be made at this time."
Parker's UK manager Spencer Brown issued a statement speaking of "total shock at the findings.
"We are just waiting to speak to Jo, (promoter) David Higgins and his promoter Frank Warren. It is very early days and we will get to the bottom of it all."
Wardley stopped fight-favourite Parker in the 11th round at London's O2 Arena in the WBO 'Interim' clash to earn the right to challenge Ukraine's undisputed world champion Oleksandr Usyk.
The British Boxing Board of Control (BBBofC), which would decide the length of any ban, could not be reached for comment on the next steps.
The UK Anti-Doping Agency (UKAD) would not comment, as is its standard practice.
British boxer Liam Cameron was banned for four years after he tested positive for benzoylecgonine, a metabolite of cocaine, in 2018 but rules have been changed since then.
Parker won the WBO heavyweight title in 2016 by defeating Mexican Andy Ruiz for the vacant belt. He lost the title to Britain's Anthony Joshua in 2018.
He had held the WBO interim heavyweight title since March last year after defeating China's Zhang Zhilei.
