British heavyweight Dillian Whyte tested positive for a banned substance before his victory against Oscar Rivas on Saturday.
If the result is confirmed, 31-year-old Whyte could face an eight-year ban, because it would be his second offence.
He served a two-year ban from 2012 to 2014 for unknowingly taking an illegal supplement.
Whyte survived a ninth-round knockdown to beat Colombian Rivas on points at London’s O2 Arena.
He is the mandatory challenger to WBC world heavyweight champion Deontay Wilder.
UK Anti-Doping (Ukad) told the BBC it would be making no comment. It is not known what the banned substance is.
Representatives for Whyte have declined to comment. He can ask for a ‘B sample’ to be analysed – but if that also tests positive he will have to request a hearing to appeal.
Whyte’s promoter Eddie Hearn has posted on social media: “Further to reports, I can confirm that both Dillian Whyte and Oscar Rivas were subject to extensive Voluntary Anti-Doping Association (Vada) and Ukad testing for their bout. Both fighters were cleared to fight by both bodies and the BBBofC.”
In a statement to the BBC on Thursday, he said: “We have learned this information like everybody else on Boxingscene.com yesterday evening. I have not heard from any concerned parties yet to get confirmation or counter.
“I will wait to know more before any more comments. Nobody from our team was aware of the situation before the fight or after the fight up to this moment.”
It is not yet known what impact this could have on Whyte’s hopes of fighting American Wilder.
In 2012, Whyte’s appeal against a ban was rejected, although a tribunal accepted his claim he did not knowingly take the banned substance.