The Israeli prime minister, Benjamin Netanyahu, who is battling for his political career in the face of various corruption allegations, has suffered a potentially devastating blow after a former confidant reportedly agreed to turn state witness.

A week after police recommended the country’s second-longest serving prime minister be indicted for bribery, Israeli press reported that Shlomo Filber would testify against his former boss to avoid jail.

Police did not confirm whether Filber, a Netanyahu appointment who headed the Ministry of Communications, would testify, but all major Israeli media reported that a deal had been reached.

Filber was arrested on Tuesday over allegations that he had promoted regulations worth hundreds of millions of dollars to the telecoms company Bezeq in return for a news website run by its principal shareholder providing favourable coverage of Netanyahu and his family.

The shareholder, Shaul Elovitch, is also in custody along with his wife and son. Former reporters at the Walla! news outlet have claimed they were put under pressure to avoid negative reports on the 68-year-old prime minister.

Elovitch has denied the allegations. Filber’s legal team has not commented.

Police also announced that they had arrested Nir Hefetz, a former Netanyahu spokesman, this week, alleging that had he tried to bribe a judge to drop a fraud case against Netanyahu’s wife.

The prime minister has not yet been named as a suspect in the case, but he is expected to be questioned. Netanyahu denies any wrongdoing and claims a media-led witch-hunt has sought to remove him from office. Appearing in a video released late on Tuesday, he said the accusations were “total madness”.

Government critics hope Filber’s testimony will open a fissure in Netanyahu’s inner circle that may force him to step down early, despite promising to remain in office until elections in 2019.

Aluf Benn, the editor-in-chief of the leftwing newspaper Haaretz, wrote a piece headlined The Final Days of Benjamin Netanyahu’s Rule. Others have speculated about snap elections, possibly as a last-ditch attempt to stall legal proceedings.

Avi Gabbay, the head of the opposition Labor party, said on Tuesday night: “The events of the last two days and recent hours make very clear: the Netanyahu age is over. We must prepare for an election soon.

“The criminal house of cards the prime minister built in recent years – corrupting the civil service, harming the rule of law, threatening freedom of the press and more than all else, dividing Israeli society – is crashing down on him and around him.”

Despite months of mudslinging, Netanyahu’s delicately balanced governing coalition has held together, but his once-sturdy political foothold appeared shaken last week when police declared they were recommending that the country’s attorney general indict him for “bribery, fraud and breach of trust” in two separate cases.

Case 1000, also known as the “gifts affair”, involves claims that he and his family received about £200,000 worth of gifts from international billionaires, including expensive cigars, pink champagne and jewellery for his wife. Alleged wealthy benefactors include the Hollywood producer and media magnate Arnon Milchan and the Australian businessman James Packer.

In return, police said, Netanyahu had helped Milchan, a producer who worked on Pretty Woman and Fight Club, with US visa matters and Israeli tax breaks.

Case 2000 relates to secret talks with the publisher of a leading Israeli newspaper, Yedioth Ahronoth, in which Netanyahu allegedly requested positive coverage in exchange for damaging a competitor, the pro-Netanyahu freesheet Israel Hayom.

The Yedioth Ahronoth columnist Nahum Barnea wrote that Netanyahu’s days in office were numbered. While he still had a legal defence, Netanyahu’s “problem is the accumulation. When so many dark clouds accumulate in the sky, the chances of rain increase,” he wrote.

“The common thread that ties together all of the investigations is the excessively long amount of time that Netanyahu has been wallowing in the swamp of governing … the less cautious he became and the greater his sense of entitlement grew. His desire to eradicate rivals by any means, his disdain for the gatekeepers, his cynicism and his self-pity superseded his good judgment and he went too far.”

Having governed for nearly 12 years over four terms, Netanyahu will now wait, possibly months, for the attorney general to make a final decision on whether to press charges.

Shortly after police released their recommendation to indict last week, a poll by Israel’s Channel 2 suggested Netanyahu could survive an election despite the corruption claims. The survey found that if an election were held, his Likud party could gain a seat. It also found that 48% of respondents believed he should quit in the wake of the allegations, and 40% said he should stay on.