JERUSALEM — A Palestinian who fasted for 66 days to protest his detention without charge ended his hunger strike on Tuesday after the Israeli authorities agreed to release him in mid-April, if no major new evidence is brought against him.

In making the deal, Israel averted the possibility of widespread unrest that many expected if the detainee, a 33-year-old member of Islamic Jihad, had died, as medical experts had determined was an imminent danger. More important, it forestalled an emergency hearing at the High Court of Justice that could have set off a broader review of Israeli military courts’ practice of administrative detention, which has been used against thousands of Palestinians over time.

Palestinian rights activists and other supporters of the detainee, Khader Adnan, insisted that the outcome remained a victory, though the case had failed to force any fundamental change in Israeli policy.

“In the end Khader’s life was saved and his message, raising awareness about administrative detention, got out to the world,” said Shawan Jabarin, director of Al Haq, a Palestinian human rights organization based in the West Bank city of Ramallah. Mr. Jabarin added that Mr. Adnan was “a hero, a champion,” and compared him to Bobby Sands, the Irish Republican Army member who died in 1981 after an equally long hunger strike.

Qadura Fares, the president of the nongovernmental Palestinian Prisoners Society based in Ramallah, said that in any case, the issue of administrative detention had come to the Israeli High Court a number of times in the past and that the judges had always accepted the arguments of the Israeli security establishment.

“We have been in that movie several times before,” said Mr. Fares, who was involved in the negotiations for the deal, communicating with the Israelis through Mr. Adnan’s lawyer.

The court had scheduled an emergency hearing for Tuesday in the case of Mr. Adnan after his condition was judged critical, but the sides canceled the petition after the deal was signed by Mr. Adnan’s lawyer, Jawad Boulus, and a lawyer for the state prosecution.

The Israeli Justice Ministry said in a statement that the deal had been reached after Mr. Adnan’s case was brought before Israel’s attorney general, attesting to the concern at the highest levels of the Israeli government about Mr. Adnan’s fate and the potential consequences. The Palestinian Authority minister of prisoner affairs, Issa Qaraqe, told the Palestinian news agency Maan that the Palestinians had asked Jordan to intervene to help save Mr. Adnan.

Palestinians have been holding demonstrations in support of Mr. Adnan throughout the West Bank and Gaza. Palestinian prisoners refused meals on Tuesday in solidarity.

The issue of administrative detention touches many Palestinian families. Israel has used the measure over the decades for periods ranging from a few months to several years. About 310 administrative detainees are in Israeli jails, down from more than 800 in January 2008.

The father of two young girls, Mr. Adnan has worked as a baker, but is also known as a leader of Islamic Jihad, an extremist organization that has carried out suicide bombings and fired rockets from Gaza into southern Israel. He has been detained several times before, mostly by Israel but also by the Palestinian Authority.

Mr. Adnan began his hunger strike on Dec. 18, a day after he was taken from his village, Arraba, in the northern West Bank, and it lasted longer than any other Palestinian hunger strike. A medical report prepared last week by an Israeli-accredited doctor on behalf of Physicians for Human Rights-Israel, and filed along with the petition to the High Court, stated that Mr. Adnan was “in immediate danger of death” and that “a fast in excess of 70 days does not permit survival.”

Israel defends its use of administrative detention as necessary for national security, and says it is used when a case is based on informants or intelligence material that cannot be revealed. Critics say the secret evidence makes it impossible for administrative detainees or their lawyers to mount a proper defense.

Administrative detention orders can be issued for a maximum period of six months, but can be renewed indefinitely. Mr. Adnan was issued a four-month detention order on Jan. 8, and it was confirmed by a military judge a month later. A first appeal was rejected on Feb. 13.

Under the terms of the deal, Mr. Adnan will be released on April 17 instead of May 8. The three-week reduction is to take into account the time that Mr. Adnan spent in interrogation after his arrest.

Israel has pledged not to renew his detention if there is no new, weighty evidence against him.

Mr. Adnan is hospitalized in northern Israel. A physician visited him after the deal was reached and confirmed that he had ended his hunger strike, Physicians for Human Rights said.

An Israeli official, speaking on the condition of anonymity, called the deal over Mr. Adnan “a workable arrangement” since ultimately he will be almost completing his four-month term of detention.

“We faced a dilemma,” the official said. “On the one hand we did not want any harm to come to him, or the wider danger in that. On the other hand it is not healthy to set a precedent that every time a Palestinian terrorist goes on hunger strike, he gets a get-out-of-jail-free pass.”