Heavyweight Robert Helenius has returned an "adverse analytical finding" following a voluntary drug test taken the day before his bout with Anthony Joshua, Matchroom Boxing says.
Helenius, 39, stepped in to face Joshua after initial opponent Dillian Whyte also returned an "adverse analytical finding" in a test before the fight.
Joshua knocked out Helenius in round seven in London on 12 August.
Matchroom were informed of the results on Friday, two weeks after the fight.
"Today, the Voluntary Anti-Doping Association (Vada) has informed Matchroom and the British Boxing Board of Control that Robert Helenius has returned an adverse analytical finding as part of a random anti-doping protocol," Matchroom said in a statement on social media.
"Matchroom defers to the relevant regulatory authorities on next steps. We vehemently continue to support voluntary anti-doping testing. We are committed to promoting a clean and fair sport for all athletes."
Helenius accepted the opportunity to face Joshua at the O2 Arena with less than a week's notice.
The Finn beat Mika Mielonen at a medieval castle in his homeland on 5 August, prior to agreeing to replace Whyte in the main event in London.
Asked by BBC Sport if boxing has as issue with doping in the build-up to the bout at the O2 Arena, two-time world champion Joshua said: "The thing is, people do wonder 'how long has this been going on?'.
"It makes me wonder 'how long has this situation been going on?'. In a way, I'm not shocked anymore in boxing.
"It's not only the belt you're competing for, it's leaving this game with your faculties. Boxing is a dangerous, dangerous game. Even on the side of it where anti-doping is involved, people do take it for granted."
For the fourth time in 12 months and the second time in as many weeks, boxing is facing serious questions about the sport's credibility and its anti-doping protocols.
Conor Benn's bout with Chris Eubank Jr was cancelled during fight week in October 2022 when it emerged Benn had failed two voluntary drug tests. Benn and Whyte both deny any wrongdoing and protest their innocence.
Amir Khan was banned for two years after an anti-doping test revealed the presence of a banned substance following his fight against Kell Brook in February 2022.