A key member of Israeli Prime Minister Naftali Bennett's Yamina party said on Wednesday she was quitting his coalition government, in a surprise move that leaves him without a parliamentary majority.
Idit Silman's announcement left Mr Bennett's coalition, an alliance of parties ranging from the Jewish right and Israeli doves to an Arab Muslim party, with 60 seats, the same as the opposition.
Although Ms Silman's defection does not mean the fall of the coalition, it raises the possibility of a return to office by veteran leader Benjamin Netanyahu, less than a year after he lost the premiership to Mr Bennett.
"I tried the path of unity. I worked a lot for this coalition," said Ms Silman, a religious conservative who served as coalition chairwoman.
"Sadly, I cannot take part in harming the Jewish identity of Israel."
On Monday, Ms Silman lashed out at Health Minister Nitzan Horowitz, after he told hospitals to allow leavened bread products on to their premises during the coming Passover holiday, in line with a recent Supreme Court.
Jewish tradition bars leavened bread from the public domain during Passover.
"I am ending my membership of the coalition and will try to continue to talk my friends into returning home and forming a right-wing government," Ms Silman said. "I know I'm not the only one who feels this way."
Mr Bennett met leaders of the coalition parties after the announcement.
"All of them want to continue with the government," he said.
"The alternative is more elections" which could lead to "dangerous instability for Israel".
The most important thing now is to stabilise the alliance, Mr Bennett said.
He accused Mr Netanyahu's supporters of pushing Ms Silman to quit through "verbal attacks" against her.
Mr Bennett's coalition may continue ruling with 60 seats, although with difficulty in passing new legislation.
But if another member of the coalition defects, the Knesset could hold a vote of no confidence and lead Israel back to the polls for a fifth parliamentary election in four years.
Political analyst Dahlia Scheindlin told AFP that if Ms Silman "is the first person to really prepare to bring down the government, she is doing it from the place of conviction".
"She is religious and I think we all underestimate the power of theology," Ms Scheindlin said.
In a formal resignation letter, Ms Silman said: "We must admit that we tried. It's time to recalculate and try to form a national, Jewish, Zionist government."
After the announcement, she was embraced by the same right-wing politicians who had relentlessly attacked her since she followed Mr Bennett into the governing coalition last year, reneging on campaign promises.
"Idit, you're proof that what guides you is the concern for the Jewish identity of Israel, the concern for the land of Israel, and I welcome you back home to the national camp," opposition leader Mr Netanyahu said in a video recording.
"I call on whoever was elected with the votes of the national camp to join Idit and come back home. You'll be received with all due honour and open arms."
Thousands of his supporters gathered on Wednesday evening in Jerusalem, shouting "Bennett, out" and demanding an end to the coalition government, AFP reported.
"Tonight, we say to the government one thing: leave," Mr Netanyahu told the crowd.
Israel's longest-serving prime minister — in office from 1996 to 1999 and again from 2009 until June — he had pledged to play the role of spoiler against Mr Bennett's government.
"There is a weak and limp government in Israel today. Its days are numbered," he said at a special session of the Knesset.
The Knesset is in recess and will reconvene on May 8 for legislative work.
"I won't name any names, but there will be more defectors," Miki Zohar of Netanyahu's Likud party told Kan public radio.
"We're in talks with more than two lawmakers who are considering coming to us."
To form a coalition of his own without new elections, Mr Netanyahu would need the support of at least 61 legislators.
He now falls well short, and does not command the support of all 60 opposition MPs.
The six legislators of the Arab-led Joint List are fierce opponents of the former premier.
Bezalel Smotrich of the Religious Zionism party, once a political partner of Mr Bennett, predicted the ruling coalition would not survive Ms Silman's defection.
"This is the beginning of the end of the left-wing, non-Zionist government of Bennett and the Islamist Movement," Mr Smotrich wrote on Twitter.