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At times, particularly with the U.S., Mr. Barak has served as a de facto foreign minister, replacing Foreign Minister Avigdor Lieberman, an ultranationalist who is often scorned in the West.

Mr. Netanyahu, meanwhile, has given Mr. Barak extra influence in decision-making out of proportion to the relatively small size of Labor.

But Labor members have grown increasingly unhappy with Mr. Barak, accusing him of enabling Mr. Netanyahu to stall in peace efforts. Although Mr. Barak is an outspoken advocate of peace with the Palestinians, he also takes a tough line on security matters and has moved slower than his critics would like on making concessions to the Palestinians.

The Labor rebels also were uncomfortable about sitting in the same government with Mr. Lieberman, who has ridiculed the notion of reaching a peace deal within the near future and openly questions the loyalty of Israel’s Arab minority.

Mr. Barak’s decision took other Labor lawmakers by surprise, and Israeli radio commentators said he orchestrated the move in tandem with Mr. Netanyahu. The prime minister’s office refused to comment.

Cabinet Ministers Isaac Herzog and Avishai Braverman quit the government just hours after Mr. Barak’s announcement.

“We must leave the government that brought us to an impasse in our aspirations to peace and forced upon us Avigdor Lieberman and his party and the foul racist discourse that threatens our democracy,” Mr. Herzog said.

Gideon Rahat, a political scientist at Hebrew University, thought the impact of Monday’s shake-up would be minimal because the departing Labor lawmakers have little influence over foreign policy.

Labor dominated Israeli politics for the country’s first three decades, producing a string of prime ministers that included Israel’s founding father, David Ben-Gurion, and the slain prime minister, Yitzhak Rabin. Mr. Barak himself briefly served as prime minister just over a decade ago.

But in recent years, Labor has been reduced to a midsize party, with just 13 seats in the current parliament. Many party members hold Mr. Barak responsible for the party’s demise, and accuse him of abandoning its socialist and dovish ideals to remain in power.

Mr. Barak’s departure from Labor resembled Prime Minister Ariel Sharon’s departure from Likud in 2005 to form the centrist Kadima Party in the wake of his pullout from the Gaza Strip. Mr. Sharon suffered a stroke shortly after, but his successor, Ehud Olmert, led the party to victory in a 2006 election.

Yohanan Plesner, a Kadima lawmaker, said it was a sad day for Israel. “This is the day the Labor Party was buried for good,” he said.

Associated Press writer Matti Friedman contributed to this report.