UNITED NATIONS — As Tony Blair was about to enter Gaza yesterday to begin the highest-level visit by a foreign diplomat since Hamas took over, the former British prime minister received a warning of a “specific threat” of assassination and quickly scrapped his plans.

For Hamas, which has taken pride in fully controlling the poverty-stricken enclave it seized last June from a rival Palestinian Arab party, Fatah, the cancellation was a disappointment. Spokesmen accused Israel of manufacturing the threat to sabotage the trip and, in effect, delaying a form of international recognition that had eluded an organization defined by America, Israel, and the European Union as a terrorist group.

The event occurred as Israel planned to receive the bodies of two Israeli soldiers, Eldad Regev and Ehud Goldwasser, in a prisoner exchange with another terrorist organization, Hezbollah, which is scheduled to take place today near the northern border town of Rosh Hanikra. Five Lebanese men held in Israeli prisons, including the convicted killer Samir Kuntar — who was officially reprieved yesterday by President Peres of five life sentences plus 47 years imprisonment — will be handed over to Hezbollah in the exchange, as well as the bodies of 199 Arab terrorists and fighters.

Despite signing the reprieve order, Mr. Peres, after meeting yesterday with family members of Mr. Kuntar’s 1979 terrorist attack victims, said, “I do not forget and do not forgive.” Mr. Kuntar, who killed a policeman before murdering in cold blood a father and his two infant daughters, is expected to receive a hero’s welcome today in a Hezbollah stronghold, the south Beirut neighborhood of Dahya.

Mr. Blair’s planned Gaza trip was not intended as a diplomatic recognition of Hamas. He had scheduled the visit to inspect Gaza projects sponsored by the steering group known as the Quartet — America, Russia, the European Union, and the United Nations — for which he serves as coordinator of activities designed to help Palestinian Arabs build state institutions. No meetings with Hamas officials were scheduled, Mr. Blair said.

“It would have been important to go and see for myself firsthand what’s happening in Gaza and I will continue to press for help for the people there,” Mr. Blair told the Ramallah-based news agency Ma’an, adding that his visit “has been postponed and not canceled.” At first he planned to overlook the threat, he told the paper, but when it clearly became “more specific and more credible,” he changed his plans.

Palestinian Arab officials immediately accused Israel of sabotage. Israel feared Mr. Blair would “witness the disaster in the Gaza Strip due to the blockade imposed on the sector and its implications and results, as well as the crimes committed by the Israeli forces,” a Hamas spokesman, Tahar An-Nunu, said.

Hamas reportedly made security arrangements in Gaza in advance of Mr. Blair’s visit, but according to Israeli reports, a last-minute warning by Israel’s internal security service, Shabak, convinced Mr. Blair that a credible threat of assassination existed. A spokesman for Mr. Blair, Matthew Doyle, told The New York Sun the trip was postponed because of “a specific threat.” He declined to comment on the source or content of the threat.

“The bilateral relations with Blair are such that we have credibility. They know we are not playing games when it comes to security,” a senior Israeli government official said, speaking on condition of anonymity. He acknowledged that Jerusalem was “concerned that a Gaza visit would be used by Hamas for propaganda purposes.” But, he added: “We never asked Blair not to go. And once we received a very specific threat, we immediately passed it on to him, and he acted accordingly.”