Dillian Whyte says he beat Oscar Rivas “fair and square” after being “cleared” to fight despite testing positive for a banned substance.
The British heavyweight, 31, defeated Rivas on points in London on Saturday but was informed he had tested positive before the bout.
Promoter Eddie Hearn has said Whyte was cleared to fight after a hearing.
Whyte tweeted: “I am so disappointed with the rubbish that has been said about me over the last few days.”
“I have lawyers dealing with it and I have been told that I can’t talk about it for good legal reasons. I was cleared to fight and I won that fight fair and square. Thanks for the support.”
Whyte served a two-year suspension from 2012 to 2014 for unknowingly taking an illegal supplement – and he could face an eight-year ban if he is found to have breached rules again.
It is not known what the banned substance is from the latest failed test.
UK Anti-Doping (Ukad) has declined to comment, while the British Boxing Board of Control underlined its policy of putting its anti-doping procedures, decisions, sanctions and suspensions in the hands of Ukad.
Hearn has said Whyte was cleared to fight Colombian Rivas at London’s O2 Arena after attending an independent panel hearing.
His Matchroom Boxing organisation has faced criticism for not informing Rivas of the failed test but the promoter has said that when a hearing cleared Whyte, he had a contractual right to fight.
The WBC, who sanctioned the bout, said it received no notification of the failed test.
Rivas’ promoter Yvon Michel told the BBC: “Nobody from our team was aware of the situation before the fight or after the fight.”
Hearn said Whyte was “absolutely broken” by the developments on Thursday and urged people to “wait for the facts” to emerge.
Whyte has 26 wins from 27 bouts and became mandatory challenger to WBC heavyweight champion Deontay Wilder with Saturday’s victory.
A bout with Wilder was mandated to take place by 18 May next year.