Conor Benn's fight against Chris Eubank Jr on Saturday has been postponed after he tested positive for a banned substance, co-promoters Matchroom and Wasserman Boxing have announced.
Eddie Hearn and Kalle Sauerland have conceded defeat in their attempts to stage the catchweight meeting at London's O2 in defiance of the British Boxing Board of Control declaring it was "prohibited" and "not in the interests of boxing".
The BBBofC acted after it was revealed on Wednesday that trace amounts of fertility drug clomiphene, which elevates testosterone levels in men, had shown up in a recent test by the Voluntary Anti-Doping Association (VADA).
The statement from the promoters described the BBBofC's decision as "procedurally flawed", claiming a legal challenge this close to the fight was not feasible.
"After discussions with various parties, we have taken the decision to formally postpone the bout between Chris Eubank Jr. and Conor Benn.
"That remains a legal issue between the promoters and the Board which we intend to pursue. However, 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," said the statement.
"As promoters, we take our obligations and duties very seriously, and a full investigation will now need to take place. We will be making no further comment at this time and news for ticket holder refunds will follow."
The bout was billed as a continuation of the Benn v Eubank feud that saw the rivals' fathers battle it out in two ferocious contests in the early 1990s.
A catchweight of 157lbs was agreed for the sons to continue the family feud into a trilogy fight, meaning Benn would have to move up two weight divisions and Eubank lose three pounds.
A new date for the fight must now be found and overseas options could be considered, with Benn, who insists he is a "clean athlete", sure to be called to a hearing at the BBBofC.
Benn said in a statement: "I am truly gutted that we are unable to make this fight happen on Saturday. And I am sorry for everyone that has been affected by the postponement.
"I am still completely shocked and surprised by this and it has been a tough couple of days. My team and I will consider the next options, including rescheduling the fight. But my immediate focus is on clearing my name because I am a clean athlete."