Heavyweight Robert Helenius says he would "never cheat" after a voluntary drug test taken before his bout with Anthony Joshua returned an "adverse analytical finding".
The 39-year-old Finn stepped in to face Joshua after initial opponent Dillian Whyte also returned an "adverse analytical finding" before the fight.
"I did not use any performance enhancement, now or ever," he said.
Joshua knocked out Helenius in round seven in London on 12 August.
In a statement on social media, Helenius added: "I have been tested my entire career.
"I was just told that I tested positive for the presence of a non-steroid substance in my system and I wanted to send an immediate direct message to Anthony Joshua and boxing fans that I didn't cheat and never would."
Promoters Matchroom were informed of the results on Friday, two weeks after the fight.
The Voluntary Anti-Doping Association (Vada) told Matchroom and the British Boxing Board of Control (BBBofC) that Robert Helenius "returned an adverse analytical finding as part of a random anti-doping protocol," Matchroom said.
"I volunteer to work with Vada and the BBBofC to do whatever it takes to clear my name," Helenius said.
He stepped in to replace Whyte at the O2 Arena with less than a week's notice, after beating Mika Mielonen on 5 August.