Vitali Klitschko, who last fought in 2004 and retired because of injuries, was due to face Jameel McCline in Munich on September 22.
But the comebacking Klitschko, once regarded as the heir apparent to Lennox Lewis, has undergone spinal surgery in an Austrian clinic, Agence France-Presse reported on Sunday.
The injury has delivered a perhaps fatal blow to his comeback efforts.
Klitschko's spokeswoman confirmed that surgery had been done on Klitschko, but did not give any details to the news service.
Klitschko was injured during a workout in Austria.
Doctors found that a disk had fallen out and decided to operate.
It looks like the end of the road for the elder Klitschko, who is 36. He last fought in December 2004, when he beat Danny Williams.
He won the right to fight for the WBC heavyweight crown in an eliminator against Kirk Johnson after Lewis stepped away from the game. He beat brother-conqueror Corrie Sanders, and then Williams.
But knee and back woes forced him to retire.
He'd hoped that his body was in form now, but it is now clear.
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