* Case centres on Israel's largest telecoms company
* Police suspect company received regulatory benefits
* Say media site gave Netanyahu positive coverage in return
* Netanyahu suspect in two other cases; denies wrongdoing (Adds comments by Netanyahu coalition partner)
JERUSALEM, Feb 21 (Reuters) - A confidant of Israeli Prime Minister Benjamin Netanyahu has agreed to turn state's witness in one of several corruption investigations posing a serious threat to the conservative leader's political survival, local media said.
Shlomo Filber's decision to testify for the state against his former boss is a dramatic turn for Netanyahu, whose inner circle had so far seemed watertight. 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".
The development has also fuelled speculation that Netanyahu, 68, will call a snap election to try to stall legal proceedings during the campaign and rally his right-wing power base behind him.
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.
Filber, who was appointed by Netanyahu to head the Communications Ministry, was arrested this week along with top executives at Bezeq Telecom, Israel's largest telecommunications company.
In unsourced reports, Israeli media said Filber has now agreed to testify for the state in the case, in which police allege that Bezeq's owners offered favourable coverage on media they controlled in return for favours from regulators. Bezeq, its owners and executives deny wrongdoing.
A spokesman for the police fraud squad declined to comment. Filber's lawyer was not immediately available to comment on whether a deal had been reached.
Police recommended last week that Netanyahu himself be indicted in two unrelated corruption investigations. The attorney-general must decide whether to accept the police recommendation to charge him.
In one he is suspected of bribery over gifts, which police say were worth nearly $300,000, that he received from wealthy businessmen. The other involves an alleged plot to win positive coverage in Israel's biggest newspaper by offering to take measures to curtail the circulation of a rival daily.
In another case revealed this week, one of Netanyahu's former spokesmen is alleged to have tried to bribe a judge to block a case against Netanyahu's wife for spending state money on personal catering. A spokesman for the Netanyahu family called the allegation "hallucinatory".
So far, partners in his governing coalition have stood by Netanyahu, saying they were awaiting the attorney-general's next moves.
"(Netanyahu) is presumed innocent until proven guilty, even if someone is trying to kick out the prime minister without elections," Agriculture Minister Uri Ariel of the Jewish Home party, said on Israel's Channel 13 after news of the state's witness deal.
Jewish Home has eight lawmakers in Netanyahu's 66-seat coalition in parliament, enough to topple the government if the party leaves.
Polls before Filber's decision suggested Netanyahu would just about survive if an election were held now, despite widespread suspicion against him.
A survey published on Feb. 14 showed that almost half of Israels electorate believe police allegations of bribery against Netanyahu, while 25 percent said they believed Netanyahu's denials. The remainder said they did not know whom to believe.
But when people were asked whether he should remain in office or temporarily step aside, 49 percent said he should stay put, while 43 percent said he should step down temporarily.
Netanyahu posted an opinion poll on Facebook on Wednesday commissioned by his Likud party that showed that it would boost its representation in parliament from 30 seats to 34 if elections were held now.
But in an analysis headlined "The final days of Netanyahu's rule", Aluf Benn, editor of the left-wing Haaretz newspaper, said his leadership "has been dealt a harsh blow, apparently a mortal one". (Additional reporting by Ari Rabinovitch and Tova Cohen; Editing by Raissa Kasolowsky and Peter Graff)