Khader Adnan, 33, a member of Islamic Jihad, has been refusing to eat since mid-December, shortly after his arrest in the occupied West Bank by Israeli forces.
West Bank officials say it is the longest hunger strike staged by a Palestinian detainee and his supporters have warned of violent reprisals against Israel if he dies.
"His health has drastically deteriorated from the last time I saw him," said his wife, Randa, who visited her husband on Wednesday, a week after the previous visit.
"I expect the worst," she says.
Adnan, who owns a bakery and a fruit and vegetable shop in his West Bank village, Arabeh, has lost 35 kg and is at risk of dying as his muscles break down, doctors said. Despite his weak state, he remains shackled to his bed, his family said.
"He insists on continuing with the hunger strike," said Randa, who is pregnant with the couple's third child.
Adnan has agreed to take minerals and glucose in his water but the Physicians for Human Rights group in Israel said this would not keep him alive and warned on Tuesday he faced "imminent death".
"Infusion of liquids, adjustment of salts, and the addition of glucose and vitamins cannot prevent certain death due to such a protracted hunger strike," the PHR said in a statement after one of their doctors saw him on Tuesday.Hospital sees 'no immediate danger'
A senior medic at the Sieff Hospital in the northern Israeli town of Safed said Adnan was in a stable condition.
"There is no immediate danger to his life, but he is at a critical point, as anyone would be after hunger striking for so long," said Dr Raymond Farah, head of the hospital's Internal Medicine Department.
Adnan is being held under so-called "administrative detention" that enables Israeli authorities to detain suspects without trial or charge indefinitely. A local rights group, al-Haq, said 315 Palestinians were being held under the edict.
The Israeli army has said in a statement he was arrested "for activities that threaten regional security".
In an open letter released Wednesday, Adnan said he was not seeking to enter the Guinness Book of Records with his action, but wanted to denounce the manner of his arrest and the subsequent detention.
"I was safe in my home and they (Israeli forces) broke into my home and terrified my daughters and my family in the same monstrous way that they do every night to dozens of Palestinians," he wrote.
Islamic Jihad, which is committed to the destruction of Israel and has launched numerous attacks on Israeli targets, has been holding daily rallies in Gaza in support of Adnan. Its leaders have threatened attacks if he dies.
There have also been demonstrations in the West Bank, but no large scale protests as of yet.
"Today I went to a demonstration ... and there were only 10 people there. I think it is because he is a member of the Islamic Jihad and not Fatah," said blogger Jalal Abu Khater.
Fatah is a secular group and the largest party in the West Bank. It is at loggerheads with the Islamic Jihad and does not share its vision of armed confrontation with Israel.
RAMALLAH (Reuters) -- The health of a Palestinian prisoner on hunger strike for 61 days to protest against his detention, has significantly worsened, his wife said Thursday after visiting her husband in an Israeli hospital.