Shimon Peres quit the Labor Party on Wednesday and endorsed his old political rival, Ariel Sharon, for reelection next spring. But Peres stopped short of joining the prime minister's nascent centrist movement.

"This has not been an easy decision for me, but I found myself faced with the contradiction between the party of which I am a member and the requirements of the political situation," Peres said at a news conference in Tel Aviv. "Without ignoring the deep connection that I have to the party's historical path and its members, I must prefer the more urgent and greater consideration."

He added, "My party activity has come to an end."

Peres's departure, the latest turn in his nearly five-decade career in Israeli party politics, leaves Labor without its most recognizable figure, and largely concludes a shift in Israeli politics triggered by Sharon's decision to withdraw from the Gaza Strip earlier this year. It also means that the next Israeli parliament, scheduled to be elected March 28, will be the first since the 1950s without Peres as a member.

His endorsement, which infuriated veteran Labor officials, could enhance Sharon's centrist credentials at a time when the prime minister is seeking recruits to his new party, known as Kadima.

The two men are friends from Israel's founding generation and have found ways to work together over the years despite sharp political differences, especially on matters of peace with the Palestinians. But it has been Sharon, the former general and architect of the settlement movement, who has tacked toward Peres, a Nobel Peace Prize laureate, in recent years.

The Kadima platform calls for the creation of a Palestinian state, but pledges to preserve Israel's major West Bank settlements and control of East Jerusalem.

"I am convinced that he is determined, as I am, to continue with the peace process and restart it immediately after the elections," Peres said of the prime minister. "I decided, therefore, to support his election and cooperate with him to realize these goals."

In engineering the pullout from Gaza, Sharon alienated the base of his Likud Party, a movement he helped found more than three decades ago as a hawkish alternative to the secular socialist Labor movement.

Sharon abandoned Likud last week, along with another 13 of its 40 parliamentary members. An opinion poll published Wednesday in Yedioth Aharonoth, Israel's largest daily newspaper, showed Likud dropping 30 seats in the next elections to become the fourth-largest party in the 120-seat parliament.

Peres brought Labor into Sharon's governing coalition a year ago to strengthen it politically for the Gaza evacuation. But his decision to remain in Sharon's government after the Gaza evacuation angered many Labor members.

The new Labor leader, Amir Peretz, declined to offer Peres a cabinet post in a future government or a guaranteed spot on its candidate list. Peres, who has twice served as prime minister, was offered the ceremonial position of party president.

Although he will not be running for parliament, Peres would likely be rewarded with a cabinet-level post in a Sharon-led government. He has reportedly been promised a role managing peace efforts with the Palestinians, or a post that would allow him to pursue his longtime interest in developing the Negev and Galilee regions.

Eyal Arad, Sharon's chief political strategist, said Peres's endorsement "certainly legitimizes those voters who have decided, or were on the verge of deciding, to leave Labor and vote Kadima."

"Just as the Likud Party after Sharon is just an empty shell, now the Labor Party remains an empty shell with Peres and his colleagues leaving," said Arad, referring to two other Labor members of parliament who have defected to Kadima.

Labor officials said bitterly that Peres would not be leaving the party had he defeated Peretz, a Moroccan-born trade union official in a party long dominated by Jews of European descent.

Ophir Pines-Paz, a Labor member of parliament and former interior minister, told Yedioth Aharonoth, "The Labor Party is committed to peace more than every other party, and Peres's attempt to excuse his abandonment by talking about peace is pathetic.

"It's too bad, because Peres did so much for the state," Pines-Paz said. "But he'll be remembered as someone who abandoned the home which he led for dozens of years for a party based on personal career interests."

Shimon Peres's decision is part of a shift in politics prompted by Israel's withdrawal from the Gaza Strip.