Prime Minister Benjamin Netanyahu's former top political partner put him on notice on Monday he would not back Israeli military "adventures", comments that appeared to caution against possible action against Iran.
Shaul Mofaz, now opposition leader, made the remarks less than a week after pulling his centrist Kadima party out of the governing coalition, where he served as vice premier for more than two months.
As a member of Netanyahu's security cabinet, Mofaz was privy to deliberations on Iran's nuclear program.
"Kadima will not embark on any operational adventures that will risk the future of our sons and daughters, and the future of Israel's citizens," Mofaz told a news conference that focused largely on a revolt inside Kadima by four lawmakers who wanted to rejoin the government.
In Israeli parlance the Hebrew term "operational" has a strictly military connotation. Mofaz is a former chief of Israel's armed forces and an ex-defense minister.
Israeli prime ministers have traditionally consulted with opposition leaders on major military operations. But in 1981 Menachem Begin ignored then opposition leader Shimon Peres's warnings against bombing Iraq's Osirak nuclear reactor.
Mofaz's opposition increases the political risks for Netanyahu in his decision-making over Iran, especially if an operation went wrong and he faced any inquiry over it later.
Recent opinion polls show most Israelis would oppose any unilateral strike on Iran.
Mofaz's comments appeared to echo those of former Israeli security officials who have publicly spoken against any go-it-alone attack on Iran's nuclear facilities, with some saying such an assault could backfire by spurring Tehran to speed up uranium enrichment.
But the failure of talks between Iran and six world powers to secure a breakthrough in curbing what the West fears is a drive to develop nuclear bombs has raised international concerns that Israel, widely assumed to be the Middle East's only nuclear-armed state, might opt for a military strike.
Those negotiations, Netanyahu said on the Fox television news channel on Sunday, had failed to slow uranium enrichment in Iran "one bit". Iran denies seeking nuclear weapons.
As he ponders his moves on the Iranian issue, Netanyahu will still have a workable parliamentary majority following Kadima's defection in a dispute over formulating a new law on ending blanket military draft exemptions for ultra-Orthodox Jews.
Kadima itself has been quickly rocked by rebellion. Mofaz said he asked a parliamentary panel to rule that party members who had threatened to bolt and rejoin the government were no longer Kadima members, effectively exiling them to back benches.
In a blow to any Likud hopes of an even wider Kadima split, the rebels had failed to recruit the minimum seven defectors required under law to form a breakaway faction that could have become a partner in Netanyahu's coalition.
Kadima is Israel's biggest party with 28 of parliament's 120 seats, but opinion polls have predicted it would fare poorly in the next election, due in late 2013.
Netanyahu could opt for an earlier poll should further cracks develop in his coalition over the contentious issue of conscription and annual political battles over the state budget.
(Additional reporting by Maayan Lubell, Editing by Jeffrey Heller and Alistair Lyon)