A last-gasp legal challenge by Israeli Prime Minister Benjamin Netanyahu to thwart a bid by a rival Rightist to head a new government was rejected on Tuesday as his opponents raced to seal a pact that would unseat him.

Naftali Bennett, Netanyahu’s former defence minister, announced on Sunday he would join a proposed alliance with centrist Opposition leader Yair Lapid, serving as its Premier first under a rotation deal.

They have until Wednesday midnight (2100 GMT) to present a final pact to President Reuven Rivlin, who handed Lapid the task of forming a new government after Netanyahu failed to do so in the wake of a close election on March 23.

Hoping to beat the deadline, Lapid, Bennett and other party leaders convened to clinch coalition agreements, sources briefed on the talks said.

In a letter to the legal counsels of the presidency and parliament, Netanyahu’s conservative Likud said Lapid was not authorised to cede the premiership to Bennett.

But President Reuven Rivlin’s office said in response that there was no legal merit to Likud’s claim because Lapid would be sworn in as “alternate Prime Minister”, second to serve as Premier as part of the rotation.

It accepted Likud’s argument that Lapid must provide the President with full details of the new government and not just announce that he has clinched a coalition deal.

The Lapid-Bennett power-share may include other Rightist politicians as well as liberal and Centre-Left parties.

Israeli media have speculated it could also court parliamentary backing from a party that draws votes from Israel’s Arab minority.