Independent medical experts have suggested that Ariel Sharon may have suffered brain damage or paralysis as a result of the heavy bleeding in his brain.
"In the event of a massive haemorrhage the prognosis is catastrophic," French specialist Prof Pierre Aamarenco told the AFP news agency but Felix Umansky, a leader of the medical team working on the Prime Minister, insisted there was still a chance that he could make a recovery.
"He can still pull through. I don't know if that will happen 100%, but I hope so," he said.
The latest operation was unexpected because doctors had said they expected no change in Ariel Sharon's condition before the weekend, and that they planned to keep him sedated in an "induced coma" until Sunday at least.
Ariel Sharon's deputy, Ehud Olmert, has taken over as caretaker prime minister.
Israeli Prime Minister Ariel Sharon has been submitted to a further brain scan, just a day after emergency surgery to stop renewed bleeding in his brain but the hospital said the scan was already planned and was not due to a change in his condition.
