The new state's witness, Shlomo Filber, was director general of the Communications Ministry from 2015 until 2017, answering directly to Netanyahu, who at the time also held the title of communications minister.
Israeli police targeted associates of Prime Minister Benjamin Netanyahu in the latest corruption investigation involving the leader and his inner circle Tuesday.
According to sources at the Securities Authority, the recordings in the end "broke" Filber, convincing him to turn state's witness.
In another case announced this week, two Netanyahu allies are alleged to have offered a judge promotion in exchange for dropping a case against the premier's wife.
Half of the respondents in a Hadashot News poll broadcast Wednesday night said Netanyahu should resign or suspend himself until his legal issues are resolved, while 33 percent said he should stay in office.
Israel's dominant political figure for a generation - in power since 2009 and for 12 years total since 1996 - Netanyahu calls the allegations against him a "witch hunt" and has said he will seek a fifth term in a national ballot due in late 2019.
Prime Minister Benjamin Netanyahu is the subject four corruption investigation.
Other leading Israeli columnists on Wednesday suggested that if Filber told all he knew, Netanyahu was probably more anxious about avoiding prison than staying in office. Speculation is now growing that he could be forced to step down or call an early election.More news: F1: Mercedes-AMG Reveals 2018 W09 EQ Power+ Championship Successor
Avi Gabbay, head of Labor Party, said he was preparing for elections. "These are not easy days".
Additionally last week police said there were grounds to indict the prime minister himself in two other cases for bribery, fraud and breach of public trust.
Israeli Attorney General Avihai Mandelblit, a Netanyahu appointee, is now set to decide if the prime minister would face official charges.
Netanyahu is accused of accepting almost $300,000 in lavish gifts from Hollywood mogul Arnon Milchan and Australian billionaire James Packer. Now, Filber may testify that he was not operating on his own behalf, but as Netanyahu's emissary.
Police have not commented on what Packer may have received, and Netanyahu has said everything he received were gifts from friends. In the other, Case 2000, he is accused of backroom dealing with the publisher of Yediot Ahronot, a major Israeli newspaper, for more favorable coverage. Filber's change of heart could leave the tough-talking Netanyahu at his most vulnerable yet, with one critic writing him off as a "political corpse".
Hefetz was arrested in a separate case known as Case 4000 on suspicion of bribery and obstruction of justice.
The middleman is also in police custody. Their alleged offer was to Hila Gerstel, a judge involved in a graft probe into Sara Netanyahu over alleged misuse of public funds.
Netanyahu swiftly and angrily denied any suggestion that he made an offer to a potential attorney general as "ludicrous".