Benjamin Netanyahu’s political opponents have moved to capitalise on a series of damning bribery indictments levelled against the Israeli leader, hoping to further weaken him at one of the lowest points in his decades-long career.

The opposition Labor party was expected to file petitions to the high court of justice to force the country’s longest-serving prime minister to step down.

Under Israeli law, a sitting prime minister is not obliged to resign when charged with a crime, only if they are convicted. But the law has never been tested as Netanyahu is the first head of an Israeli government to face criminal charges while in office.

The indictments have thrown Netanyahu’s personal fate into uncertainty at a time when he also faces deep political pressure. He has failed twice this year to form a new government despite two national elections and the prospect of a third now looms as Netanyahu’s popularity is expected to have taken a hit. Cracks in his Likud party have already begun to show.

Amir Peretz, the chair of the Labor-Gesher alliance party, said the courts should order Netanyahu to declare a leave of absence since he was only an interim leader after failing to form a government.

“Labor-Gesher will argue that given the fact that the prime minister heads a transitional government, which did not gain the confidence of the Knesset, he must declare himself incapacitated as the law requires of government ministers,” Peretz said.

The main opposition Blue and White party, which finished near neck and neck with Likud but has also failed to form a government, announced on Friday that while Netanyahu was not legally required to step down as prime minister, he should immediately vacate other cabinet posts. The 70-year-old is also minister for agriculture, health, social affairs and diaspora affairs.

A statement from Blue and White said it had instructed its lawyers to approach both Netanyahu and the attorney general, Avichai Mandelblit, adding it had legal precedent from a previous supreme court ruling.

Mandelblit’s announcement on Thursday evening that Netanyahu was indicted in three cases, including on charges of fraud and breach of trust, and one serious case of bribery, appeared to deeply shake Israel’s longest-serving prime minister.

The dramatic 63-page indictment accused Netanyahu of accepting hundreds of thousands of pounds in luxury gifts from billionaire friends and for giving favours to Israeli media and telecoms magnates in exchange for positive news coverage.

He lashed out hours later, arguing in a televised speech that the charges amounted to an “attempted coup” against him. “The public has lost trust in the legal system,” he added.

Traditional allies, including many rightwing nationalists and religious parties, issued statements in support of their embattled leader, calling for the public to wait until the court case to end before making judgment. That process could take years.

Ahead of his indictment on Thursday, however, and in a clear challenge to his boss, a senior Likud lawmaker called for a leadership primary, and put his name forward as a candidate.

Gideon Sa’ar, a Netanyahu rival who has long been tipped to succeed him, said that if the country goes to a third round of elections, then Likud members should reconsider who leads them.

“I think I will be able to form a government, and I think I will be able to unite the country and the nation,” he said at the Jerusalem Post newspaper’s diplomatic conference.