The Free Samer Issawi Campaign said on Monday on Twitter that the Palestinian prisoner, who has been on hunger strike for seven months, is "currently dying" in Israeli prisons.
"The longest hunger strike in history and the world doesn't say a thing about it," the campaign tweeted.
Issawi has stopped drinking water, taking vitamins and boycotted all medical tests after refusing food, Lebanon's Al-Akhbar newspaper had reported a few days ago.
The news outlet added that a doctor has informed Issawi that his heart could stop functioning "at any time" due to water shortage in his body.
Issawi was released as part of the Egyptian-brokered prisoner swap deal between Hamas and Israeli authorities in October 2011.
That deal led to the release of Israeli soldier Gilad Shalit in exchange for more than 1,000 Palestinian prisoners.
However, Issawi was rearrested on 7 July, 2012 under the so-called administrative detention law. The law, which has been in place since the end of the British mandate in Palestine in 1948, legalises the arrest of Palestinians if they are a "threat" to Israel's national security.
Amnesty Internationl demanded that Israel either charge or release immediately those interned and end such practise, which allows for the indefinite renewal of arrest orders for six months at a time.
The Palestinian hunger striker is suffering various health problems, including weight loss, general weakness, protein shortages, sugar shortage, deteriorated visions, and others
A Palestinian source told the German Press Agency (DPA) last month that Egypt is in talks with Israel over Palestinian prisoners on hunger strike based on the request of the West Bank-based Palestinian Authority, which called on Cairo to intervene diplomatically to secure the release of the prisoners. But no actual results have been seen until now.
The scope of the hunger strike has posed a new challenge to Israel, which has come under international criticism over detentions without trial and could face a violent Palestinian backlash if any of the prisoners die.
Earlier this week, Akram Al-Rikhawi, a Palestinian prisoner who last year staged a hunger strike, was freed from an Israeli prison and returned to his home in the Gaza Strip.