Israeli Prime Minister Ariel Sharon is having emergency surgery after his condition worsened and his life is in danger, doctors say.
Ariel Sharon has been in a coma since 4 January
Mr Sharon, 77, has been in a coma at the Hadassah hospital in Jerusalem since 4 January, when he suffered a severe stroke and underwent surgery.
Doctors decided to operate on Saturday after they discovered his digestive tract had suffered serious damage.
Interim Prime Minister Ehud Olmert assumed his powers in January.
"Sharon's life is in danger," a spokeswoman for the Jerusalem hospital told the Associated Press.
His condition is now "very serious, or critical," Yael Bossem-Levy said.
Relatives and staff have been arriving at the hospital following news of Mr Sharon's deterioration.
Doctors carried out an abdominal scan on Saturday morning, which prompted the move to take him into surgery at about 1100 (0900 GMT).
ARIEL SHARON'S HEALTH
18 Dec 2005: Sharon suffers minor stroke
4 Jan 2006: Sharon rushed to hospital one day before scheduled heart surgery with major stroke
4/5 Jan: Undergoes two operations overnight
6 Jan: Third round of surgery
9 Jan: Doctors start to try to rouse him from medically induced coma
15 Jan: Has tracheotomy to help to wean him off respirator
25 Jan: Medical team hold talks with long-term care specialists
1 Feb: Doctors insert feeding tube into stomach
10 Feb: Brain scan shows no change in condition
11 Feb: Has emergency surgery after condition worsens
The check showed problems with the blood flow to his intestines and damage to his digestive tract, Ms Bossem-Levy said.
The hospital will not give an update on his condition until the end of surgery, which could take between three and six hours.
Mr Sharon is in a frail condition after several operations following his stroke.
Doctors had inserted a feeding tube into his stomach earlier this month.
Mr Sharon's failure to wake up after being taken off sedatives last month has led to speculation that he has suffered severe brain damage.
His deputy Ehud Olmert will lead the newly-formed centrist Kadima party in the general election on 28 March.
Mr Olmert has been Mr Sharon's closest political ally for most of the past decade and is seen as the politician most likely to continue his legacy.
He has said that if he wins next month's poll, Israel will retain West Bank settlement blocs and Jerusalem.
However, he also said that Israel would be prepared to give up parts of the West Bank where most Palestinians were living.
Israeli leaders say they will not deal with a Palestinian government including Hamas, following the Islamic militant group's sweeping victory in polls last month.
Israel regards Hamas as a terrorist group committed to the destruction of the Jewish state.
The political leader of Hamas says the group is ready to talk to Israel, but will not renounce violence.