Hisham Abu Hawash, a 40-year-old member of the Islamic Jihad movement, began refusing food in August to protest Israel holding him without charges or trial.
The married father of five from Dura in the south of the Israeli-occupied West Bank was being held under administrative detention, a practice of arresting suspects for renewable six-month terms without allowing them to view the charges or evidence against them.
"His condition is difficult and complex," Liad Aviel, spokesman of the Shamir Medical Centre in central Israel where Abu Hawash is being held, told AFP.
The International Committee of the Red Cross said medical teams that visited Abu Hawash had found him "in critical condition requiring expert clinical monitoring".
According to the ICRC, Abu Hawash has been refusing food for around 140 days.
It issued a statement warning of "potentially irreversible health consequences and possible tragic loss of life".
His wife, Aisha Hrebat, told AFP on Sunday he was in a "very dangerous" situation, adding that "since yesterday he can't talk at all and doesn't know what's going on around him".
"Even after he ends his strike, he will have difficult problems," she said, adding their lawyer was submitting an urgent appeal against his detention to Israel's supreme court.
An Israeli security source described Abu Hawash as "an Islamic Jihad operative, who was arrested due to involvement in terror activity".
Israel says the protocol prevents crimes while authorities continue to gather evidence, though Palestinians say it denies them of their rights.
"The way Israel has used administrative detention is arbitrary," said Shawan Jabarin, head of the Al-Haq rights group based in Ramallah on the West Bank.
He said Abu Hawash was one of about 550 Palestinians held by Israel in administrative detention.
Abu Hawash's plight has ignited Palestinian support.
Palestinian civil affairs minister Hussein al-Sheikh called on Israel via Twitter to "release Abu Hawash immediately", with his appeal echoed by protesters who gathered over the weekend in Ramallah.
At a rally in Gaza, Ismail Radwan, an official with the Islamist movement Hamas that rules the enclave, said Israel must understand that prisoners constitute a "red line" for the Palestinians.
The Islamic Jihad, the second largest militant group in Gaza, said it holds "the Israeli occupation fully responsible for the deteriorating health" of Abu Hawash, threatening revenge if he dies.