Joseph Parker has denied taking any banned substances and is "confident" he will be cleared after failing a drugs test.
Parker is facing a two-year ban after a routine test given to the Voluntary Anti-Doping Agency returned traces of cocaine.
The test was conducted on the day of the New Zealand heavyweight's fight against Fabio Wardley, which the Briton won last month.
"Before my recent fight I took a voluntary test and have now been informed that it returned an adverse result. This came as a real surprise to me," Parker said in a statement on Instagram.
"I did not take any prohibited substance, I do not use performance enhancing drugs and do not support their use. I am cooperating fully with the process now under way, and I am confident the investigation will clear my name.
"Thank you to everyone who has sent messages of support. It means a great deal to me and my family. When the investigation is complete, I will speak openly and answer questions."
The British Boxing Board of Control [BBBofC] has told SportNews it is "investigating" the matter.
Parker, 33, is a former world champion, who lost his belt to Anthony Joshua in 2018, but he went into his fight against Wardley in arguably the best form of his career.
After a punishing loss to Joe Joyce in 2022, Parker won his last six contests prior to Wardley, picking up victories over Deontay Wilder and Zhilei Zhang, as well as winning the WBO Interim strap.
He was due to challenge then-IBF belt-holder Daniel Dubois in February, only for Dubois to fall ill just days before the fight was meant to take place.
Parker put his position as the WBO mandatory challenger to undisputed champion Oleksandr Usyk on the line by taking the fight against Wardley.
It was a gamble that backfired and news of his failed drugs test now leaves major questions over his future in the sport.
SportNews Boxing expert Andy Scott:
"The headline is shocking.
"Everybody loves Joseph Parker. I think he's most people's second-favourite heavyweight. But there were questions as to why take that fight [against Wardley]?
"He was in line to fight [Oleksandr] Usyk. He instead took a risky fight against Wardley, and now this is the headline.
"Not only did he lose the fight, he now has this investigation that he will have to come out of and clear his name.
"For context, Liam Cameron is the high-profile case in boxing with similar circumstances. He received a four-year ban for traces of cocaine, but the rules have changed since then.
"It will be interesting to see how this develops further. There's a full investigation, we'll wait to see what the outcome is of that, but there is a possibility that he's facing a two-year ban."
