Hisham Abu Hawash, who has been protesting his detention by Israel by refusing food for around 140 days, is facing "potentially irreversible health consequences and possible tragic loss of life".
Palestinian prisoner Hisham Abu Hawash has been languishing in hospital "in critical condition" after an extensive hunger strike teetered close to death.
His wife, Aisha Hrebat, said on Sunday that he was in a "very dangerous" situation. "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.
40-year-old Abu Hawash began refusing food in August to protest Israel holding him without charges or trial.
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".
The father of five from Dura in the south of the Israeli-occupied West Bank was being held under administrative detention.
That is a practice of arresting suspects for renewable six-month terms without allowing them to view the charges or evidence against them.
Israel says the protocol prevents crimes while authorities continue to gather evidence, though Palestinians say it denies them their rights.
"The way Israel has used administrative detention is arbitrary," said Shawan Jabarin, head of the Al Haq rights group based in Ramallah in the occupied 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.