Israel’s defense minister, Ehud Barak, broke away Monday from the left-leaning Labor Party he had led and formed a smaller centrist faction that will stay in the governing coalition under a new name. The surprise move shook up Israeli politics but was expected to have little impact on its policies.

Mr. Barak said at a news conference that Labor had drifted too far from the mainstream and that he was tired of the pressure to withdraw from the government over the lack of progress on peace. Mr. Barak and four like-minded Labor members of Parliament will remain alongside Prime Minister Benjamin Netanyahu of the hawkish Likud Party; the remaining eight Labor dissidents will become part of the opposition.

With those departures, Mr. Netanyahu’s majority in the 120-member Parliament will drop to 66 members, from 74. But without the threat of Labor’s departure, the government is now considered to be more stable than before, not less. Mr. Barak informed Mr. Netanyahu in advance of his planned break with Labor, and was assured that he would remain chief of the Defense Ministry.

“Some of the members of the faction wanted to see the Labor Party as a militant socialist left-wing party,” Mr. Barak and his four colleagues said in a statement announcing the formation of their new movement, which is to be called Independence. “We supported the government’s efforts to advance an arrangement with the Palestinians and neighboring countries, but we did not think it proper to stand with a stopwatch regarding this complex process. We also did not accept the self-flagellation of those who see the State of Israel as exclusively responsible for the lack of a diplomatic process.”

Mr. Barak and his colleagues added that pressure within Labor “created among the Palestinians the illusion that the government was about to fall, and that it would be desirable and in their interests to wait for a more ‘suitable’ government.”

Within Labor, there had been talk for months of leaving the coalition because of its lack of progress with the Palestinians. Hostility toward Mr. Barak in the party ranks was palpable, and efforts to oust him had been widely aired.

Isaac Herzog, who resigned Monday as welfare minister, and Avishai Braverman, who quit as minority affairs minister, will vie for leadership of the newly consolidated and more frankly left-wing Labor Party.

“The Labor Party, which founded the State of Israel, got rid of the hump on its back,” Mr. Herzog said at a news conference. “Ehud Barak’s masked ball is over.” He added, “I am confident today more than ever that the Labor Party has again become a political home for those who felt betrayed by it.”

Labor and its predecessors were the dominant political force in Israel for much of the nation’s 62-year history. But failed peace efforts, shifting demographics, the Palestinian uprising a decade ago and the takeover of Gaza by Hamas began a rightward turn among voters that has been reflected in numerous opinion polls.

The inclusion of Labor in the otherwise right-wing government had provided it with something of a buffer internationally. Mr. Barak has been a frequent visitor to Washington in the 22 months of the government’s existence.

American efforts to arrange direct peace negotiations between Israeli and Palestinian leaders and an agreement on the creation of a Palestinian state have not succeeded, and few analysts are optimistic about a breakthrough. The Palestinians want Israel to freeze construction of West Bank settlements before returning to the talks; the Israelis reject that condition as unacceptable. They had put in place a partial freeze in what they considered a good-will gesture, but it ended in September.

Einat Wilf, one of the four Labor members of Parliament who joined Mr. Barak in the Independence faction — the others were Matan Vilnai, Shalom Simhon and Orit Noked — said in a telephone interview that the new government arrangement would send a signal to the Palestinians that there was little point in waiting for the Netanyahu government to fall apart.

“I don’t belong to the camp that believes Israel is solely responsible for the failure of these negotiations,” Ms. Wilf said. “The Palestinians bear responsibility for not entering the talks. Some people have sent them a message to wait around for a new government.”

Saeb Erekat, the chief Palestinian negotiator, dismissed the notion that he and his colleagues had been waiting for a new Israeli partner. “We know this government is strong, and we have never spoken about its composition or the need to change it,” he said by telephone. “These people are hallucinating.” He added that the Palestinians planned to ask the United Nations Security Council to condemn Israeli settlements in the West Bank in the next week or two.

Tzipi Livni, the leader of the Israeli opposition and the center-left Kadima Party, said the political shift should lead to new elections. But Mr. Netanyahu said his government was not going anywhere. “The government has grown much stronger today — in its governance, in its stability,” he said, “and this is important for Israel.”