Former Israeli Prime Minister Ariel Sharon, comatose since a 2006 stroke, is in a critical, life-threatening condition from which he is unlikely to rally, a hospital official said today.

"I am no prophet, but the feeling of his doctors and his sons ... is that there has been a change for the worse," Zeev Rotstein, director of the Sheba Medical Center, told reporters.

Rotstein, in the first official medical statement on Sharon's condition after reports yesterday that he suffered a kidney malfunction, said doctors expect a deterioration in several life-sustaining organs.

"We are defining his condition as critical, and there is definitely a threat to his life," he said. "The feeling of everyone ... is that this decline is very serious."

Sharon, 85, an ex-general and right-wing leader, has been in a vegetative state since suffering a stroke in January 2006.

An MRI scan a year ago detected some brain activity, when Sharon was shown photographs of his family and also when asked to imagine his home. But neurologists who carried out the study said be was effectively in a locked in state, unable to activate any muscles.

Once a hard-line defence chief, reviled in much of the Arab world, Sharon made a dramatic political about-face with a unilateral pullout in 2005 from the Gaza Strip.

His illness came shortly after he quit the right-wing Likud party to found a centrist faction in the hope of advancing peace moves with the Palestinians.