Hadassah hospital spokesman Ron Krumer said a CT scan had revealed "serious damage to the prime minister's digestive system", prompting doctors to operate.

Yael Bosem-Levy, a hospital spokeswoman, said Sharon's condition had deteriorated to its most critical point since his admission. She said at 11:30 am local time yesterday that surgery could take three to six hours.

Sharon's death will almost certainly leave Ehud Olmert, named interim prime minister after the 77-year-old leader's brain haemorrhage on 4 January, in charge until Israel's national election, only six weeks away.

Olmert has stepped swiftly into the shoes of the former general who dominated the Middle East scene for decades, pledging to press ahead with Sharon's tough security policies and threatening to set Israel's final borders unilaterally if peacemaking with the Palestinians remained frozen.

Opinion polls predict the centrist Kadima party, which Sharon founded after a rebellion in his right-wing Likud over Israel's Gaza pull-out last summer, will easily win the 28 March general election with Olmert at its helm.

Government officials said Olmert was receiving updates of Sharon's condition and family members and senior advisers rushed to the hospital on news of his deterioration.

After suffering his massive stroke, Sharon was put into a medically-induced coma. Doctors later tried without success to rouse him. Earlier this month, they inserted a feeding tube into his stomach in preparation for possible transfer to a long-term care facility, underscoring medical experts' beliefs that he would never recover.

Long reviled in the Arab world but increasingly regarded as a peacemaker by the west, Sharon suffered his stroke at a crucial point in Israeli politics, as he was fighting for re-election on a promise to end conflict with Palestinians.

In recent years, he has voiced support for a Palestinian state but demanded the disarming of Palestinian militant groups before peace talks could resume.

Olmert has hammered home that demand, part of a US-backed peace 'road map' that also calls on Israel to stop Jewish settlement expansion, with added force since the Islamic militant group Hamas won the 25 January Palestinian election.

If Sharon dies, the Israeli cabinet will meet to designate an 'acting prime minister' until next month's election; Olmert would be the near certain choice.

Jewish law generally requires burial within 24 hours of death, but Sharon's funeral could be delayed so he can lie in state.