JERUSALEM — A former prime minister, Ehud Barak, announced yesterday his much-anticipated return to politics, saying he will seek the leadership of the Labor Party in the first step toward a possible bid for the country’s highest office.

Mr. Barak’s announcement immediately shook up the race for the top spot in Labor, Prime Minister Olmert’s main coalition partner. Labor primaries are set for May.

Mr. Barak, 63, spent nearly six years in political exile after he was crushed by Ariel Sharon in a 2001 election. The defeat followed a short and stormy tenure as prime minister that abruptly ended after failed efforts at making peace with Syria and the Palestinian Arabs.

The former army commando and chief of staff said he would run against the party’s current leader, Defense Minister Amir Peretz, whose popularity has plummeted following Israel’s inconclusive summer war with Hezbollah guerrillas in Lebanon.

Mr. Barak spent months laying the groundwork for a comeback, gaining the support of key members of the party and establishing himself as the early favorite.

The next leader of the Labor Party will also likely become defense minister. Mr. Barak’s military pedigree could give him an advantage as Israel recovers from the war while facing a potential nuclear threat from Iran.