Avichai Mandelblit was set to give a statement at 1730 GMT announcing whether he would indict Netanyahu on a range of graft allegations he denies, a justice ministry statement said without further details.
The decision is expected to have a wide-reaching impact not just on the embattled leader but on Israeli politics in general, as the country has been without a government for nearly a year due to political infighting.
Neither Netanyahu nor his centrist rival Benny Gantz have been able to form a coalition government following deadlocked elections in September, with the country edging closer to a third election within twelve months.
An indictment might permanently damage 70-year-old Netanyahu's political career and cause allies to break away from him, whereas a reprieve would likely give him a new lease of life.
Rightwinger Netanyahu, who has been in power since 2009, is Israel's longest-serving prime minister and dominates the country's political scene.
Range of allegations
In February, Mandelblit had announced his intention to indict Netanyahu on charges of fraud, breach of trust and bribery, following up on police recommendations.
In May he extended until October a deadline for Netanyahu's pre-indictment hearing but rejected a request for a 12-month delay.
The allegations against the PM range from receiving gifts worth thousands of dollars to a deal to change regulatory frameworks in favour of a media group in exchange for favourable coverage.
Netanyahu has vehemently denied all the allegations, calling the corruption investigation a "witch-hunt" and alleging it has been motivated by his enemies' desire to force him from office.
Of the investigations against Netanyahu, the third, known as Case 4,000, is seen as the most serious.
He is alleged to have negotiated with Shaul Elovitch, the controlling shareholder of Israeli telecommunications giant Bezeq, to get positive coverage on his Walla! news site in exchange for policies benefiting Bezeq.
Mandelblit said in February he intended to indict Netanyahu for bribery, fraud and breach of trust in this case.
New elections loom
The announcement comes as the country's political system has ground to a halt.
Earlier Thursday Israel's President Reuven Rivlin turned to the country's parliament in the hope of avoiding a third election in 12 months.
Following the near neck-and-neck polls in September Netanyahu and Gantz were given four weeks each to try and form a new government.
Netanyahu, head of the Likud party, was given first go but failed. And Gantz, who leads the Blue and White coalition, admitted defeat late Wednesday after a similar period.
Rivlin has now given parliament 21 days to find a candidate who can command the support of the majority of the country's 120 MPs.
If not, a new general election will be called for early 2020.
Rivlin, who has been urging a compromise to break the political deadlock, told MPs they had a responsibility to keep the country running.
"Your political fate is not more important than the fate of an old lady in a hospital," he said.
Netanyahu and Gantz, a former military general, had been discussing forming a unity government alongside former defence minister Avigdor Lieberman's Yisrael Beitunu party.
But talks broke down late Tuesday, with the two men arguing over who should go first if they were to rotate the premiership.
All sides say they oppose new elections, which are unpopular with the Israeli public, but have traded blame over the faltering talks.
Despite having failed in previous attempts, both Netanyahu and Gantz could be nominated in the next three weeks.
Mandelblit, supposedly a non-political figure, could have the biggest sway in the future not just of Netanyahu but of the country's politics.
Addressing Gantz Thursday morning, Netanyahu urged him to come to "personal, immediate negotiations, you and I, without preconditions.
"We can get over the differences. The state is important to all of us."
But Israeli media said in reality the talks were deadlocked and the parties were already preparing their campaigns for new elections.