Tamir Pardo says he was concerned that he had been ordered to prepare for an illegal war on Israel's arch-foe
Israeli Prime Minister Benjamin Netanyahu gave the order in 2011 to prepare a military strike against Iran, according to a former Mossad chief.
Tamir Pardo, who headed the Israeli spy agency Mossad from 2011 to 2016, made the claim in an interview with the Israeli Keshet TV investigative show Uvda.
His comments were released ahead of a broadcast of the show expected to air on Thursday evening.
According to Pardo, the spy chief was ordered to ready forces for an attack on Iran within 15 days, and was told that the command was not a drill.
"These things have enormous significance," Pardo told the Israeli show.
"When he tells you to start the countdown process, you know that he isn't playing games with you."
Pardo's claims come as tensions continue to run high between Iran and Israel after Israeli jets targeted several military bases in Syria thought to hold Iranian soldiers.
He alleged that when receiving the command to prepare the attack, he demanded "clarifications" on who "authorised" the order to "launch a war", concerned he was heading into an illegal operation.
"I checked with legal advisers, I consulted with everyone I could to understand who is authorised to give the order concerning launching a war," said Pardo.
The Israeli prime minister's office has not commented on the claims.
Former prime minister Ehud Barak, who served as Netanyahu's defence minister, said in 2015 that Netanyahu had attempted to bomb Iran in 2010 and 2011, but was opposed by senior Israeli officials.
Israel has grown concerned about the increasing Iranian presence in neighbouring Syria, where Iran has helped Syrian President Bashar al-Assad battle armed rebel groups.
Earlier this week, Elaph, a Saudi-owned news site, claimed that Iran and Israel had engaged in indirect negotiations over fighting in southwest Syria.
Iran reportedly pledged to stay out of fighting in southwest Syria between Syrian forces and rebel groups while Israel said it would not intervene in battles near the Israeli-occupied Golan Heights or near the Jordanian border, so long as Hezbollah and Iranian-backed militias are not involved.
Earlier this year, Israel shot down a suspected Iranian drone that entered the Israeli-occupied Golan Heights. Tel Aviv responded by attacking anti-aircraft positions in Syria, and an Israeli warplane was shot down during the clashes.