"In the last few days, we have seen a gradual decline in the functioning of Ariel Sharon's vital organs, which are essential for his survival," Professor Ze'ev Rotstein, the head of Sheba Medical Center at Tel Hashomer said.
"His state is classed as critical, meaning his life is in danger. The medical staff and Sharon's family are expecting a turn for the worse," Rotstein said.
Sharon, who was reported to have been suffering from serious kidney failure, was not on dialysis, but that doctors were administering antibiotics due to numerous infections, Rotstein told reporters.
Sheba Medical Center spokesman Amir Marom was quoted by the Jerusalem Post as saying that 85-year-old Sharon's condition had deteriorated over the past few days. Asked what was expected for the day ahead, he said, "Only God knows."
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Sharon's two sons, Omri and Gilad, remained close to their father late yesterday awaiting further developments and treatment.
Doctors at the hospital reportedly told Gilad and Omri that their father had no more than a few days left to live.
The 11th Prime Minister of Israel has been in a coma since his January 4, 2006, stroke. Sharon's family intended to mark the anniversary of the stroke this week.
The former premier has mostly undergone care at Sheba Medical Center at Tel Hashomer since then.
Sharon has seen sporadic positive responses to scans that measure brain activity, but has remained in a deep vegetative state since his initial hospitalisation.
Having fought in three Middle Eastern wars, Sharon is admired by many Israelis as a great military leader, but reviled by Palestinians.
While serving as defence minister in 1982, he masterminded Israel's invasion of Lebanon. During the invasion, Lebanese Christian militiamen allied to Israel massacred hundreds of Palestinians in two Beirut refugee camps under Israeli control.
He was nevertheless elected prime minister 18 years later, pledging to achieve "security and true peace", and served until his second stroke.