Lamont Peterson's camp is "disappointed and distraught" by the decision to cancel a world light-welterweight title showdown next week against Britain's Amir Khan after the American failed a drug test.
But an angry Khan labelled Peterson "a cheat" and called for the return of his International Boxing Federation and World Boxing Association titles.
In a statement released on Thursday by Peterson's team, the reigning WBA and IBF champion lamented the wipeout of a planned May 19 rematch of his controversial victory last December.
"Team Peterson is very disappointed and distraught by the decision to cancel the May 19 rematch against Amir Khan," the statement said. "This is an extremely difficult decision to accept."
Peterson underwent medical tests and was examined by doctors this week in hopes of providing data to the Nevada State Athletic Commission ahead of the fight in Las Vegas.
"As a condition for this rematch Lamont demanded that Olympic-style random drug testing be implemented," the statement said. "He has been a true advocate for making boxing drug free and fair.
"In his 18-year career (10 amateur and 8 professional) Lamont Peterson has never failed a drug test and has always complied with the rules beyond this isolated and explainable occurrence.
"We still stand behind the fact that he did nothing wrong and he was more than ready to go through with the May 19 fight.
"We will vigorously pursue the truth with regards to this matter and continue to fight to protect this young man's character, credibility and all he has accomplished. Once all the facts have been reviewed we have no doubt that he will be vindicated."
However, an angry Khan said: "The Nevada Commission have taken the licence off Lamont Peterson and they don't want him to fight in the next fight.
"So I think whoever I fight next they'll put the titles on the line or we still go through the process and we might get the titles back and the fight that we had last time will be a no-contest."
Khan, who said Peterson's test had come as a "big shock", added: "The truth's come out now and it just proves that Lamont Peterson was a cheat really.
"It hurt me more than anything because I really did want to go in there and put on a performance and show the world how good Amir Khan is.
"I'm already at the right weight and it was a tough, tough training camp.
"I was so fit and looking forward to this fight. I wanted this fight even when I heard he failed his drugs test.
"I did everything to get the fight on, but my parents and my team said to me 'Look, at the end of the day Amir, it could be a risk to your life.
"It's not going to be fair to step into the ring with someone who is gonna be on drugs and gonna be cheating'.
"The next fight for me will be June, July. I've got a meeting with Golden Boy Promotions over the next few days and hopefully we'll set a date."
Peterson beat Khan in Washington, the American's hometown, in December but referee Joe Cooper controversially deducted two points from Khan for excessive pushing.
Without those deductions, the fight scoring would have resulted in a draw and Khan would have kept his titles.