Benny Gantz, a popular centrist member of Israel's three-member War Cabinet, has threatened to resign if Benjamin Netanyahu fails to present a post-war plan for Gaza.
He has given the prime minister a deadline of June 8 to organise the plans. If Mr Gantz were to resign, it would leave Mr Netanyahu more reliant on far-right allies.
His announcement deepens the divide in Israel's leadership more than seven months into a war in which Israel has so far failed to achieve its goals of dismantling Hamas and returning the scores of hostages abducted by the militant Palestinian group.
Mr Gantz outlined a six-point plan that includes the return of hostages, ending Hamas' rule, demilitarising the Gaza Strip, and establishing an international administration of civilian affairs with American, European, Arab, and Palestinian cooperation. The plan also supports efforts to normalise relations with Saudi Arabia and expand military service to all Israelis.
"If you choose the path of fanatics and lead the entire nation to the abyss — we will be forced to quit the government," he said.
In a statement, Mr Netanyahu said that Mr Gantz had chosen to issue an ultimatum to the prime minister instead of to Hamas, and called his conditions "euphemisms" for Israel's defeat.
Mr Gantz, a longtime political rival of Mr Netanyahu, joined his coalition and the War Cabinet in the early days of the war as a gesture of national unity. His departure would leave Mr Netanyahu more dependent on far-right allies who advocate for Israel to occupy Gaza and rebuild Jewish settlements there.
Mr Gantz spoke just days after Israeli Defence Minister Yoav Gallant, the third member of the War Cabinet, stated he would not remain in his post if Israel chose to reoccupy Gaza, and called on the government to plan for a Palestinian administration.
In what was seen as a swipe at Mr Netanyahu, Mr Gantz said, "personal and political considerations have begun to penetrate into the holy of holies of Israel's security." Mr Netanyahu's critics accuse the prime minister of seeking to prolong the war to avoid new elections, allegations he denies.
Polls suggest Mr Gantz is the most likely candidate to be the next prime minister, a change that could expose Mr Netanyahu to prosecution on longstanding corruption charges.
