Klitschko (left) and Chisora had been due to meet on 11 December
IBF and WBO heavyweight champion Wladimir Klitschko has called off his fight with British boxer Dereck Chisora for a second time because of injury.
The bout against unbeaten Chisora, 27, was originally scheduled for December but was postponed when Ukrainian Klitschko tore an abdominal muscle.
The meeting was rearranged for 30 April in Germany's SAP Arena in Mannheim.
"I am very sorry for this final cancellation especially for the fans," said 34-year-old Klitschko.
Britain's WBA heavyweight champion David Haye hopes to take on Wladimir Klitschko in July before meeting his brother Vitali - the WBC champion - in the Londoner's final fight before retiring ahead of turning 31 in October.
Wladimir's doctor, Dr. Hans-Wilhelm Mueller-Wohlfahrt, said: "At present the exact timeframe for recovery is difficult to predict. A fight in April would be too great a risk."
Klitschko added: "I am sure that we will soon carry out a [rescheduled] event in Mannheim."
But Klitschko did not state whether that would be against Chisora or Haye.
The news comes days after it emerged Haye was back in talks over a 2 July unification showdown.
Haye had previously doubted Klitschko's ability to fight Chisora on 30 April and then meet him barely two months later.
Unless Klitschko reschedules the Chisora fight for a third time - which seems unlikely - that issue appears to have been overcome, giving him plenty of time to get in shape for a huge showdown with long-time rival Haye providing the stomach injury heals.
Chisora and his promoter, Frank Warren, declined to comment on the cancellation.