Israel's High Court on Monday said it upheld the new government's bid to increase the number of ministers, allowing a crucial bill to go before parliament later in the day.

The opposition Yesh Atid party had sought an injunction preventing the first reading of a bill to permit enlargement of the cabinet from its present statutory maximum of 18 ministers, arguing that it was unconstitutional.

The court "rejected the demand by Yesh Atid for an injunction against the amendment to the law", a court statement said.

Getting the legislation through will be the first of many likely challenges for Prime Minister Benjamin Netanyahu's fledgling coalition, which has a razor-thin majority of 61 votes in the 120-seat parliament, the Knesset.

"The coalition and the opposition are gearing up today for an exhausting political battle which will serve as the first test of... Netanyahu’s leadership," the mass circulation Yediot Aharonot newspaper wrote on Monday.

"This is a dramatic test for the coalition that will show how it will be able to work within the narrow format of just 61 MPs."

Debate on the first of the bill's required three readings is expected to begin at 1300 GMT, but opponents of the amendment have pledged to stage filibusters to draw out the process.

"I hope (your reporter) brings a sleeping bag," Itzik Shmuli of the centre-left Zionist Union told Israeli army radio Monday.

"The opposition will be given time for debate," Knesset speaker Yuli Edelstein told the same radio station.

"I don't think that this legislation will be completed by tomorrow," he added.

- Major concessions -

"There are 59 opposition MPs -- I shall give them time both today and tomorrow to express their reservations."

But Edelstein also said that time would not be unlimited.

"We have to form a government in Israel. If there is a majority it will be formed; if not then apparently somebody else will need to get the task of trying," he said.

Netanyahu's alliance was built over weeks of intensive negotiations following a March general election, at the cost of major concessions to his coalition partners and resentment within his own party.

The amendment aims to let him give more portfolios to senior members of his rightwing Likud.

Knesset rules and procedure are also raising obstacles to passage of the amendment.

The normal procedure for legislation requires that a bill, after passing a first reading, goes to the parliamentary law committee for further consideration.

Edelstein, a member of Likud although his role as speaker obliges him to be strictly nonpartisan, said the newly installed Knesset had yet to form such a committee and one would have to be set up.

Also, he added, the government had not yet submitted the required cost estimate with its draft.

In the March 17 election Likud won 30 seats. His partners are the centre-right Kulanu (10 seats), the far-right Jewish Home (eight) and the two ultra-Orthodox parties Shas (seven) and United Torah Judaism (six).

The law capping the number of ministers was approved last year in an attempt to reduce public spending, at the behest of Yesh Atid leader Yair Lapid, who was finance minister at the time.