James Toney has been stripped of his WBA heavyweight title victory after failing a post-fight drugs test.
The American beat John Ruiz on points on 30 April, but was relieved of the win by the New York State Athletic Commission on Wednesday.
The Commission changed the verdict to a no contest, suspended Toney for 90 days and fined him $10,000 (£5,500).
Under WBA rules, the title could now revert back to Ruiz, who ended his brief retirement earlier this week.
Ruiz's attorney told fightnews.com: "We are waiting for the WBA to immediately implement its rule.
"It's as clear as it can be. We're glad the truth has come out and Johnny hasn't lost his championship."
NYSAC chairman Ron Scott Stevens would not say what banned substance had been detected in Toney's drugs test.
"This is a medical issue and it would be a breach of his confidentiality and the privacy that we owe him," he said.
Toney's promoter, Dan Goossen, issued a statement on behalf of the boxer denying he had knowingly taken any performance-enhancing drug.
"I've never used any illegal substances to prepare myself for a fight," said Toney's statement.
"Being accused of taking performance enchancing substances is an insult to me. I don't do drugs. Period."
Goossen added: "Toney received medical treatment for recovery from his biceps and triceps surgery last year.
"His doctor has stated that the combination of medications used to control the inflammation and tissue growth caused the positive test result."
Stevens said he believed it was the first time New York had ever changed a title fight result because of a drug violation.
He said an appeal against the decision could be lodged within 30 days, which would set up an open hearing.