The fight between Conor Benn and Chris Eubank Jr, due to take place in London on Saturday, is postponed following the former returning an “adverse analytical finding”.
It comes after Benn, 26, returned what promoters said was an “adverse analytical finding for trace amounts of a fertility drug”.
The British Boxing Board of Control had said the bout was “prohibited and not in the interests of boxing”.
Promoter Matchroom confirmed on Thursday that the fight had been postponed.
“After discussions with various parties, we have taken the decision to formally postpone the bout between Chris Eubank Jr. and Conor Benn,” Matchroom said.
“It is undeniable that the British Boxing Board of Control’s decision to withdraw their sanctioning was procedurally flawed and without due process,” it added.
“That remains a legal issue between the promoters and the Board which we intend to pursue.”
At the media workout on Wednesday, Benn insisted he was a “clean” fighter.
Eubank and Benn were scheduled to meet in the ring at a catchweight of 157lbs, 30 years after their fathers Nigel Benn and Chris Eubank Sr fought.
Matchroom promoter Eddie Hearn had been seeking to overturn the BBBofC’s decision as both fighters’ camp said in a statement on Wednesday they wanted the fight to go ahead.
Earlier on Thursday Hearn ruled out seeking a new body to sanction the fight.
“It’s been done before and it’s not something I’ve looked at before or something I necessarily agree with,” Hearn said. “This is not a situation where we want to run rogue and come up with an alternative plan.”
The Matchroom statement said that “whilst there are legal routes to facilitate the fight taking place as planned, we do not believe that it is in the fighters’ interests for those to be pursued at such a late stage, or in the wider interests of the sport.”
It added: “As promoters, we take our obligations and duties very seriously, and a full investigation will now need to take place.”
Benn returned an “adverse analytical finding” for the fertility drug clomifene in August, following a test taken by the Voluntary Anti-Doping Association (Vada).
Clomifene can be used to boost testosterone levels in men and is banned inside and outside competition by the World Anti-Doping Agency (Wada).
Vada was hired by Benn and Eubank to provide extra testing alongside UK Anti-Doping, which is the Board’s officially anti-doping partner and adheres to Wada.
Eubank’s promoter Kalle Sauerland confirmed on Wednesday that both camps had known of the drug test results for several weeks and Eubank wanted to go ahead with the fight.
“We discussed it straight away with the most important person from our side, the athlete,” Sauerland said. “He was happy to continue and spoke to Benn directly.”
Internally at Matchroom and their broadcasters DAZN, they are convinced Benn is innocent of intentionally doping.
Hearn argued that because the Board do not “recognise” VADA, they had no reason to call off the fight or suspend Benn.
UK Anti-Doping (UKAD) Chief Executive, Jane Rumble said: “UKAD notes the reporting of a failed VADA test by the boxer Conor Benn with concern.”
Rumble added: “UKAD acts on all reports of doping and always encourages anyone with information on suspected doping activity to come forward and share that with us.”