However the Israeli Prisons Authority denied the inmates were back on strike, saying they had food in their cells and were eating.
But in reference to the 800 Palestinians in Ashkelon prison, Isa Qaraqi, head of the Palestinian Prisoners Club said: "They resumed (fasting) today".
"Israel must sit with the prisoners and negotiate with them over their demands. The continuation of the strike is dangerous. Maybe there could be instances of death in the prisons and it would be a big catastrophe," Qaraqi said.
Around 2800 prisoners, seen by Palestinians as symbolic of resistance to Israeli occupation, have been refusing food to demand wardens stop strip searches, allow more frequent family visits, improve sanitation and install public telephones.
Israeli officials dismiss the liquids-only fast, which began on 15 August, with 1500 hunger strikers, as a ploy to secure easier communication with militant groups waging a nearly four-year-old revolt in the West Bank and Gaza Strip.
The Prisons Authority has denied reaching a deal with the Ashkelon hunger strikers, but admitted it was in dialogue with them.
An Authority spokesman said: "They refused to take meals today as some form of protest. But they have food in their cells from the canteen. They are eating." He said more than 40 hunger-striking women had also broken their fast.
Qaraqi said previously the Ashkelon prisoners had reached a partial agreement with prison officials for a halt to strip searches and for family visits during which prisoners would be allowed to hug their children.
The prisoners had expected to hold talks with prison officials on Monday, but Qaraqi said a meeting was abruptly cancelled by the Israeli side.
More than 7000 Palestinians, excluding common criminals, are held in Israeli jails.