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The two main challengers to Israeli Prime Minister Binyamin Netanyahu have announced an alliance on Thursday in a bid to defeat him in April's election.

Benny Gantz, a former Israeli military chief of staff, and "centrist" politician Yair Lapid said they would form a joint list for the April 9 election and take turns as prime minister if they won.

The announcement comes after Netanyahu reached a preliminary electoral deal with fringe far-right religious-nationalist parties, led by disciples of the convicted terrorist Rabbi Meir Kahane, awarding the education and housing ministries to the out-and-out racist Otzma Yehudit party - Arab-haters who believe in Jewish supremacy.

Kahane's Kach Party, the forerunner of Oztma Yehudit, was after a string of deadly militant attacks deemed too extreme even for Israel's Knesset and was proscribed as a terrorist organisation by Israel, the United States and the European Union.

Gantz, a political newcomer whose first major political speech was on January 29 this year, heads his recently launched Israel Resilience party, while Lapid leads Yesh Atid, which currently holds 11 of the 120 seats in parliament.

Opinion polls have shown the two to be the main challengers to Netanyahu, who is never-the-less expected to win - despite wide-ranging corruption investigations into his affairs.

Gantz, a career soldier whose post-military life saw his law enforcement technology company shut down after its principal Russian investor was the target of US sanctions, has faced repeated barbs from the prime minister for being a "weak leftist".

But when appointing him as chief of military staff in 2011, Netanyahu had said Gantz was "an excellent officer and experienced commander, and had rich operational and logistical experience" following his varied roles in the military occupation of Palestinian territory and in wars against Lebanon.

In a campaign speech in Tel Aviv on Tuesday night, Gantz accused the prime minister of becoming "addicted to the pleasures of power, corruption and hedonism".

Former defence minister Moshe Yaalon - who once described Palestinians as a "cancer" and peace activists as a "virus" - is running as Israel Resilience's number two.

Gabi Ashkenazi, who as military chief of staff commanded Israeli troops in Operation: Cast Lead - killing 1,400 Palestinians in Gaza in 2008-09 - will also join the new alliance.

"Out of a sense of deep national responsibility, Benny Gantz, Yair Lapid and Moshe Yaalon decided on the creation of a unified list which will serve as Israel's new party of government," the statement from Yesh Atid said.

"The party will put forward a new leadership team which will guarantee the security of Israel and will reunite the divided elements of Israeli society."

Netanyahu has been prime minister for around 13 years in all and currently leads what is seen as the most right-wing government in Israel's history.

His Likud party issued a grim warning following the merger.

"The choice is clear: it's either a left-wing Lapid-Gantz government with the support of the Arab parties, or a right-wing government led by Netanyahu," a statement said.

On Thursday, the lead editorial in Israel's Haaretz newspaper read: "Under the sponsorship of a prime minister who is prepared to sacrifice every principle and smash every institution in his battle to entrench his regime, the followers of Kahane will return to the Knesset riding like the Messiah on the donkey of religious Zionism."

Thursday night is the deadline for submitting party lists for the election. Left-wing groups and Arab-led parties are still negotiating over possible mergers.