Chris Eubank Jr v Conor Benn has been called off following the latter's positive drug test.
Organisers had hoped to find a way to go ahead with Saturday's pay-per-view showdown, but promoters Matchroom admitted defeat on Thursday afternoon, less than 24 hours before the scheduled weigh-in.
The British Boxing Board of Control had declared that the fight was “prohibited” and “not in the interests of boxing” after Benn had returned an “adverse analytical finding for trace amounts of a fertility drug”, only for Matchroom to insist that the fight would go ahead.
It soon became clear that could not happen and a statement from Eddie Hearn's company read: "After discussions with various parties, we have taken the decision to formally postpone the bout between Chris Eubank Jr and Conor Benn.
"It is undeniable that the British Boxing Board of Control’s decision to withdraw their sanctioning was procedurally flawed and without due process. 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.
"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."
Earlier in the day, Eddie Hearn used social media to rule out appointing an overseas governing body to sanction the fight but he retained hope that it could still be saved.
That prospect looked bleak, however, when the head-to-head press conference in a Canary Wharf hotel scheduled for lunchtime was pushed back twice and Benn left the hotel where he had been based with his bags packed. The undercard fighters followed soon after.
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.