Jarrell Miller says he has “done nothing wrong” and will appeal against the decision to deny him a licence to face Anthony Joshua in New York.
The New York State Athletic Commission refused him a licence to fight on 1 June after he returned an “adverse finding” in a drug-testing sample.
“The facts will prevail and I shall be vindicated,” said Miller, 30.
“My team and I stand for integrity, decency and honesty and we will fight this with everything we have.”
Miller returned the adverse finding on March 20 and his team have now requested his B sample be tested.
But the decision by NYSAC – who sanction the bout – has currently left Joshua without an opponent for his US debut at Madison Square Garden.
Miller, who was due to step up in class and face his toughest test to date, will also lose out on a reported purse of about £5m.
“I am absolutely devastated upon hearing the news my boxing license has been revoked in NY State and I will be vigorously appealing this decision,” he added in a post on Instagram.
“I have never knowingly taken any banned substance and when I found out the news last night I was totally shocked.
“This was a voluntarily test that I was very happy to do and these results came just one week after another voluntarily test that I had taken which was completely clean.
“I refuse to just lie down and let my dream be taken away from me when I know in my heart that I’ve done nothing wrong.
“I don’t have anything to hide and the truth will make itself known.”
Joshua’s camp say they are in the process of searching for a replacement for the fight night, with American Michael Hunter mooted as a potential opponent.
Hunter, 30, has 16 wins and one loss – to Oleksandr Usyk at cruiserweight – on his record.
Joshua’s promoter Eddie Hearn believes there are “eight acceptable guys” who could step in, including Cuba’s Luis Ortiz and Poland’s Adam Kownacki.