The British Boxing Board of Control says Saturday’s fight between Conor Benn and Chris Eubank Jr is “prohibited and not in the interests of boxing”.
Benn, 26, has returned an “adverse analytical finding for trace amounts of a fertility drug”, promoters have said.
They say he has not been suspended and is free to fight Eubank, 33, in London as his B sample has yet to be tested.
But British boxing’s governing body has issued a statement that places the all-British fight in doubt.
The board said its decision not to sanction the fight was communicated to the boxers and promoters on Wednesday morning.
Benn said he had spoken to Eubank, they had both taken medical and legal advice and “want the fight to happen for the fans”.
“He believes in me. It’s not who I am, it’s not what I’m about. He was understanding,” Benn added.
“Throughout my whole career I’ve never had any issues before. I’m a clean athlete and we’ll get to the bottom of this.”
Eubank said he was ready to fight, although added: “There was no phone call. We haven’t spoken since this has been announced.”
Benn’s promoter Eddie Hearn, of Matchroom Boxing, told BBC Sport: “As we stand right now, the British Boxing Board of Control is not sanctioning the fight.
“That does not mean the fight is off, but there is a process we have to go through.
“Conor Benn is not suspended, he is free and clear to fight. There’s lots going on with the lawyers.”
A statement earlier on Wednesday from promoters Matchroom and Wasserman Boxing said Benn had not been charged or suspended and that he and Eubank are still keen to fight.
The pair are scheduled to meet in a catchweight bout at the O2 Arena, 30 years after their fathers, Nigel Benn and Chris Eubank Sr, faced each other in the ring.
The promoters’ statement said: “We have been made aware that a random anti-doping test for Conor Benn conducted by the Voluntary Anti-Doping Association returned an adverse analytical finding for trace amounts of a fertility drug.
“Mr Benn has since passed a doping control test conducted by the UK Anti-Doping Agency, the anti-doping authority to which the British Boxing Board of Control has delegated its doping control testing for the bout. Mr Benn has passed all doping control tests conducted by Ukad.
“Both fighters have taken medical and legal advice, are aware of all relevant information and wish to proceed with the bout this Saturday.”
It is conceivable the promoters could find another body to sanction the bout.