Prime Minister Benjamin Netanyahu, tasked three weeks ago with forming Israel's next government, fell short Wednesday of striking a coalition with rightwing partners after reports emerged of talks with a centre-left party.
Netanyahu's rightwing Likud party won Israel's March 17 election but without a majority, and President Reuven Rivlin gave him until April 22 to form a coalition, as stipulated by the law.
If he fails, Netanyahu can request an additional two weeks.
Public television reported on Monday that Netanyahu and Isaac Herzog, who heads the Zionist Union and was Netanyahu's main challenger in the vote, had met secretly to discuss the possibility of a national unity government.
Both factions denied such a meeting had taken place.
Herzog has not explicitly ruled out a unity government but said his party would form part of the opposition.
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For his part, Netanyahu has said he would not form a unity government but instead bring his "natural" rightwing and ultra-Orthodox partners into a coalition.
But a Wednesday report in Maariv newspaper quoted a Zionist Union official as confirming the Netanyahu-Herzog meeting.
MPs from the Zionist Union were "surprised and angry", he said.
Political analysts say the reported meeting could spur the ultra-Orthodox, centre and rightwing parties currently negotiating with Netanyahu to reduce their demands and join his coalition.
"If the demands and lack of decency of the natural partners will continue and Likud is pushed to a corner, Likud will not hesitate to approach Herzog to offer a unity government, or even call new elections," firebrand party member Miri Regev wrote on her Facebook page.
Coalition talks have reportedly reached an impasse over ministerial roles for Naftali Bennett, the far-right Jewish Home head, and Avidgor Lieberman, who leads the ultra-nationalist Yisrael Beitenu.