Palestinian and Israeli rights groups on Sunday wrote to EU foreign policy chief Catherine Ashton demanding her "urgent intervention" on behalf of 125 prisoners on long-term hunger strike.
The letter was sent as the overall number of Palestinian prisoners refusing food climbed to 290, including 70 being treated in hospital, an Israel Prisons Service spokeswoman told AFP.
Of that number, 125 have been on hunger strike for more than five weeks, beginning their mass protest on or shortly after April 24, Palestinian rights groups say.
Most of the prisoners are administrative detainees who are refusing to eat in protest over their being held without trial in a procedure which can be extended indefinitely.
"We ... wish to bring to your attention the ongoing mass hunger strike involving approximately 125 Palestinian detainees and prisoners, and request your urgent intervention on their behalf," said the letter, signed by 17 rights groups and the Palestinian prisoners' affairs ministry.
"As of June 1, the majority of the hunger strikers have gone without food for 38 days.
"We have reached a critical stage and unless there is immediate intervention there will be dire consequences for the health of all those on strike," it said.
Many had stopped taking vitamins and were only drinking water, it said, accusing Israel of withholding salt from them for the first fortnight of their strike.
The IPS denied the allegation, with spokeswoman Sivan Weizman telling AFP the detainees had been given "everything as required by law".
About 5,000 Palestinians are being held in Israeli prisons, nearly 200 of them under administrative detention orders, which allow suspects to be jailed without trial for up to six months.
Such orders can be renewed indefinitely by a military court.