JERUSALEM, Jan 17 (Reuters) - Israeli Defense Minister Ehud Barak will break away from the Labor party in a move on Monday that will split the once-dominant center-left movement, but which did not appear likely to topple the governing coalition.

Barak's surprise step was announced by a parliamentary official who said the Labor leader wrote in a letter that he was taking four of the party's 13 legislators with him into a new faction.

Barak had been under pressure within Labor to press Prime Minister Benjamin Netanyahu's right-leaning government to move more quickly towards an elusive peace agreement with the Palestinians. He also faced a future party leadership challenge.

The inclusion of Labor in Netanyahu's administration broadened its political base and was widely seen as softening its right-wing image internationally.

In a separate statement, Barak's deputy, Matan Vinai, said that leaving Labor would enable the new faction members to pursue peace without a stopwatch.

Political analysts said Barak was expected to remain in the coalition, but it was unclear whether Labor's eight remaining parliament members would continue to support the government.

Netanyahu currently has a 74-member majority in the 120-seat parliament including the Labor party. (Writing by Ari Rabinovitch, Editing by Jeffrey Heller)