Here the Planet Sport examines the situation so far.
What has happened?
Fight preparations were thrown into turmoil when it was revealed on Wednesday that a number of weeks ago Benn had retuned an adverse finding for the banned substance clomiphene in a test conducted by the Voluntary Anti-Doping Association (VADA). The British Boxing Board of Control (BBBofC) responded by declaring the fight "prohibited", but co-promoters Matchroom and Wasserman Boxing have been lobbying behind the scenes to have the fight staged as scheduled. On Thursday afternoon they conceded defeat.
What is clomiphene?
A fertility drug, clomiphene has the capacity to significantly elevate testosterone levels in men. It is banned in and out of competition. Benn was 146.5lbs when he crushed Chris van Heerden in April and was due to fight Eubank at the catchweight of 157lbs.
Was postponing the right decision?
Having called off the fight, promoter Eddie Hearn said: "I hope you feel that the correct action has been taken." It was the only course of action - the question is why did it take so long? While Matchroom and Wasserman Boxing attempted to salvage the contest, boxing's battered reputation was being picked apart by the mere consideration of allowing a fighter who has tested positive to enter the ring.
What happens now?
An immediate refund has been promised to fans who have paid for tickets, pointing to the prospect of the fight being rescheduled overseas, possibly in Abu Dhabi or Germany. Hearn is determined for it to go ahead and both fighters want the pay day and the opportunity to continue their fathers' rivalry from the early 1990s.
Could Benn be in trouble?
Benn is likely to face a disciplinary hearing at the BBBofC and possibly UK Anti-Doping, but legal action could delay that in time for him to fight elsewhere. The 26-year-old is not currently suspended, a point Hearn has repeatedly made.