The move stands to complicate an already formidable process facing Netanyahu of blending two vastly different sectors of Israel's population into a government after the polls.
Leaders of the Shas and Torah Judaism factions told Israeli media they would combine forces to resist a powerful new centrist party that seized second place in Tuesday's poll on a campaign pledge to deny fervently religious communities traditional perks such as mass exemptions from military duties.
"We intend to unite into one negotiating team," Rabbi Ovadiah Youssef, told members of his Shas party, of a plan to combine the two parties' 18 seats in the 120-member parliament, to compete with political newcomer Yair Lapid's, whose Yesh Atid or "There is a Future" party won 19.
Moshe Gafny, veteran lawmaker with the Torah Judaism party, also said, "we intend to forge a common bloc", the Ynet news website and the Haaretz website said.
Their remarks came a day after Netanyahu launched informal talks with Lapid, but it may take weeks until a government is formed to succeed the current right-wing coalition.
Formal negotiations may only be launched once President Shimon Peres designates a prime minister, with Netanyahu an obvious choice, a step anticipated in a matter of days, after Peres meets party leaders.
Lapid, a former television star now seen as a senior partner of Netanyahu, campaigned on a promise to change policy on the highly emotive draft issue in a country where most men are called up for compulsory military service at 18.
He has also pledged to improve the lot for Israel's tax-burdened middle class including by forcing more of the ultra-Orthodox to join the work force instead of living off stipends for religious study.
While also calling for a renewal of peace talks with the Palestinians frozen since 2010, such issues took a back seat in Lapid's election campaign to bread-and-butter economic concerns.
The ultra-Orthodox have long rejected any changes in draft or labour policies for their constituents as a threat to their beliefs, though their political leaders have also not ruled out Lapid as a governing partner, suggesting they may compromise.
Netanyahu, whose party won 31 seats on Tuesday is also seen as partnering with the 12-seat far-right Jewish Home or "Bayit Yehudi" faction along with Lapid's party, which would give him a majority of 62 in Israel's parliament.
Although he can technically rule without the help of ultra-Orthodox allies, Netanyahu is seen as likely to try to include them as well, to cushion his majority and enjoy the freer hand they tend to give him on foreign and security policy matters.
JERUSALEM (Reuters) -- Israeli Ultra-Orthodox parties said on Friday they would band together to fight for inclusion in any coalition government forged by Prime Minister Benjamin Netanyahu after his rightist Likud party's narrow win in a Jan. 22 parliamentary election.