Saul 'Canelo' Alvarez has withdrawn from his world middleweight title rematch against Gennady Golovkin while he awaits the outcome of a hearing into an adverse drug test.
Mexican Alvarez, 27, twice tested positive for clenbuterol in February.
The first bout in September ended in a controversial draw, with the rematch scheduled for 5 May in Las Vegas.
Promoter Eric Gomez said it was "extremely unlikely this matter is going to be resolved" in time.
"At this point we are unfortunately going to have to cancel," he added.
"As you all know there is a hearing date of 18 April. We need enough time to promote a fight of this magnitude."
WBA, WBC and IBF middleweight world champion Golovkin, 35, said Alvarez's replacement will be named soon.
He tweeted: "I will be defending my titles on Cinco de Mayo (5 May) in Las Vegas! Official announcement coming soon!"
Clenbuterol is used to treat asthma but can also help build lean muscle mass and burn off fat. It is sometimes illegally mixed into livestock feed to make meat leaner.
Earlier this year, Alvarez said he was "surprised and bothered" by the adverse test findings and that he was "an athlete who respects the sport".
Alvarez's promoters blamed "meat contamination" for his positive tests on 17 and 20 February and said contamination had "impacted dozens of athletes in Mexico over the last years".
Daniel Eichner, director at the World Anti-Doping Agency-accredited laboratory that conducted the tests, said at the time: "These values are all within the range of what is expected from meat contamination."
Kazakh Golovkin is undefeated in 38 professional bouts, while Alvarez has won 49 and drawn two of his 52 fights. His only defeat came against Floyd Mayweather in 2013.
'It would have been the biggest bout of 2018' - analysis
BBC Sport's Luke Reddy
The irony in much of this is that Canelo's trainer Chepo Reynoso used to be a butcher, so you would expect errors linked to the country's history with this problem to be ironed out.
It truly is a loss for boxing as every single second of their first encounter was captivating. Canelo counter-punched brilliantly, Golovkin attacked relentlessly and the controversial draw paved the way for what would have been the biggest bout of 2018.
One very interested party in this development will be Britain's WBO world middleweight champion Billy Joe Saunders.
He has sought a Golovkin unification fight in the past and Golovkin himself has spoken of his interest in the contest.
Saunders is set to face Martin Murray in a rescheduled bout on 23 June but expect him to now press hard for the chance to face the Kazakh for all four belts before the year is out.