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religious leaders, including key election allies of , criticised Tuesday a ending an Orthodox monopoly on Jewish conversions. 

Late Monday, the court ruled that people whose conversion to Judaism in Israel was overseen by a reform or conservative rabbi must be considered Jewish and therefore entitled to Israeli citizenship. 

Previously, the Jewish state only recognised conservative and reform conversions that took place outside Israel.

Within the country, the stricter Orthodox movements had to sanction conversions for them to be considered legitimate. 

While it was not immediately clear how many people the ruling would affect, the decision has been perceived as a challenge to the Orthodox movement's control over determining who is a Jew.

"Someone converted by the reform (movement) and the like are not Jews and no supreme court decision will change that fact," Israel's Chief Rabbi David Lau said in a statement.

Three weeks ahead of elections, backlash from stricter Jewish groups could create political complications for Netanyahu, who has no clear path to re-election without the ironclad support of religious parties.

A lawmaker from the ultra-Orthodox Shas party, Yoav Ben Tzur, said Tuesday that party leader and Interior Minister Aryeh Deri had submitted a bill to parliament that would effectively overturn the court's decision. 

Election threat

The bill seeks to legally affirm that only institutions acting on behalf of Israel's Orthodox rabbinate could conduct conversions, Ben Tzur told Army Radio. 

He added that unless the prime minister agreed to back it, Shas would not join a Netanyahu-led coalition. 

"We shall not sit in any government which does not pledge to amend the (existing) conversion law," he said.

Most polls show Netanyahu's right-wing Likud party winning 28 or 29 seats in the 120-member parliament, the Knesset.

The premier will need the backing of the two main ultra-Orthodox coalitions, Shas and United Torah Judaism - slated to win a combined 15 seats - among other parties, if he hopes to form a 61-seat majority.

Netanyahu did not immediately comment on the supreme court ruling, which put an end to a protracted legal battle, but a Likud statement backed the Orthodox position.

The premier's political opponents were quick to side with the court. 

"I applaud this decision, which acknowledges the diversity of Jewish identity and opts for the wise course of inclusiveness," said Defence Minister Benny Gantz, Netanyahu's bitter rival.

Labour Party candidate and reform rabbi Gilad Kariv called it a "landmark" decision. 

Agencies contributed to this report.