Yuval Diskin, who retired as Shin Bet chief in 2011, last year warned that Netanyahu sought a "messianic" war with Iran.
His new criticism was potentially more potent given that Shin Bet duties include vetting civil servants' psychological fitness for security posts, though there was no indication Diskin was citing any official assessment of the prime minister.
"At play inside Netanyahu, in my opinion, is a mix of ideology, a deep sense that he is a prince of a 'royal family' from the Jerusalem elite, alongside insecurity and a deep fear of taking responsibility," Diskin said in a front-page interview published by Israel's biggest-selling daily Yedioth Ahronoth.
Ridiculing what he deemed Netanyahu's "zig-zag" style in decision-making, Diskin added that the prime minister "has no strong core, no tough kernel about which you can say, 'Know what? In an extreme situation, in a crisis situation, I can follow him. I can trust him.'"
Yedioth, which has often editorialized against Netanyahu, published a response by the prime minister's office dismissing Diskin's remarks as "fatuous". The statement accused Diskin of being motivated by politics and resentment at not having been chosen to head the Mossad spy service when he left the Shin Bet.
An Israeli official, who declined to be named, played down the impact of Diskin's remarks.
The Shin Bet does not conduct psychological "profiling" of prime ministers, the official said, predicting that the public would perceive Diskin as having voiced his own opinion only.
A Diskin confidant, speaking on condition of anonymity, confirmed to Reuters that he had sought the Mossad post, but argued that the former Shin Bet chief saw in it the opportunity "to ensure the right decisions are made on national security".Iran looms
Topping these concerns is the Iranian nuclear program, in which Israel sees a mortal threat and against which Netanyahu has threatened military strikes.
Israel's threat has complicated diplomatic efforts to find a solution with Tehran which denies it is seeking to make nuclear weapons and has pledged wide-ranging reprisals if attacked.
Opinion polls show that while only around a fifth of Israelis would support a unilateral war on Iran, Netanyahu is still strongly favored to win a third term.
Maariv daily on Friday predicted his party list would take 36 of parliament's 120 seats in this month's ballot, positioning him to build another conservative coalition government.
Addressing the United Nations in September, Netanyahu set out a mid-2013 "red line" for Iran's uranium enrichment, signalling that Israel was postponing any military action.
Many experts question whether Israel, even though it is widely reputed to have the region's sole atomic arsenal, is capable of taking Iran on alone. Some see Netanyahu's years of sabre-rattling as a bluff.
Keeping the issue simmering, the prime minister told Israeli diplomats on Thursday: "Our commitment was and remains preventing Iran from attaining nuclear weaponry."
Other retired Israeli national security figures have joined Diskin in criticizing the Netanyahu government. At least one of them, former top general Gabi Ashkenazi, had his tenure in uniform cut short in a spat with Defense Minister Ehud Barak.
JERUSALEM (Reuters) -- The former head of Israel's internal security service said Prime Minister Benjamin Netanyahu on Friday was wavering and weak in an unusually personal condemnation of the right-winger, who enjoys a strong lead in polls ahead of the Jan. 22 election.