Hughie Fury will not be fit to fight Michael Hunter on October 29 due to suffering ongoing symptoms of long Covid.
The fight, which was due to be shown live on Sky Sports, had already been rescheduled from early July due to Fury suffering with illness.
In a statement, Fury said he has been advised by doctors to rest, after not boxing for over a year.
"I am a professional boxer who needs to fight in order to pay my bills and to live," he said.
"But I have been out for over a year now and, to my frustration, I have been advised by doctors to take total rest at the moment because I'm suffering symptoms of long Covid.
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"I feel weak with no strength. I've had all sorts of checks and they've diagnosed me as having long Covid symptoms.
"It is frustrating but there is nothing I can do if my body's not allowing me to train. I'm going to take time out for the rest of this year and pick up again next year when I've rested properly.
"I have had 29 bouts, I'm ready for big fights and my promoter has guaranteed me a big fight upon my return. If that's Michael Hunter next, that's fine. If it's someone else, that's fine as well. I think everyone knows I will fight anyone as I have done over the years."
The 28-year-old has fought top-class heavyweights throughout his career, boxing Kubrat Pulev, Alexander Povetkin and Joseph Parker.
But this fight with Hunter would have been a key moment for him. It was an eliminator for the WBA belt held by Daniel Dubois and the winner would have been in the frame for major fights in the heavyweight division.
Instead, Fury will have to put his progression on hold while he recovers. It means he will have been out of action for more than a year, since stopping Christian Hammer inside five rounds in his last fight in 2021.
But he is determined to return to the ring in 2023.
His father and trainer, Peter, added that Fury is "sapped of all strength" and he is "taking complete rest away from boxing for the rest of this year".
"Life brings all sorts of obstacles and boxing is no different," the coach said. "Health is more important than money or trying to keep everyone happy.
"Hughie will fight as soon as possible next year and the team he has behind him guarantees him a big fight next year.
"If Michael Hunter is available and still wants this fight, that will be Hughie's next fight. If not, Hughie will fight another serious fight.
"Hughie will be back next year and he is just one fight away from mixing it with the top heavyweights out there."