JERUSALEM (Reuters) - Israel is quietly preparing for the possibility of a nuclear-armed Iran despite public pledges to deny its arch-foe the means to pose an "existential threat", Israeli political and defence sources said on Thursday.

They said Prime Minister Ehud Olmert has instructed cabinet officials to draft proposals on how Israel, whose security strategy is widely assumed to hinge on having the Middle East's only atomic arsenal, might deal with losing this monopoly.

Israel predicts that Iran's nuclear programme could produce warheads by 2009. Western intelligence services say it may take several more years.

Tehran denies seeking the bomb but its open hostility to the Jewish state and speculation about Israeli or U.S. pre-emptive strikes on its nuclear sites have stirred fears of regional war.

Olmert has endorsed U.S.-led efforts to curb Iran's atomic ambitions through sanctions. He has also hinted that Israel, which bombed Iraq's nuclear reactor in 1981 and is believed to have carried out a similar raid against Syria in September, could hit Iran too if it deems diplomatic pressure a dead end.

But two senior Israeli sources with knowledge of the Olmert government's defence planning said a memorandum was being prepared for "the day after", in case Iran attains nuclear weapons.

"There are long-term ramifications to be addressed, like how to maintain our deterrent and military response capabilities, or how to off-set the attrition on Israeli society that would be generated by fear of Iranian nukes," one source said.

An Israeli government spokesman declined comment.

Ami Ayalon, a minister in Olmert's security cabinet, refused to discuss classified policy-making in an interview with Reuters but said Israel should pursue a three-pronged strategy on Iran.

"First, we must make clear that this is a threat not just to Israel, but to the wider world. Second, we must exhaustively consider all preventive options. And third, we must anticipate the possibility of those options not working," Ayalon said.

"MUSLIM BOMB"

Iranian President Mahmoud Ahmadinejad has called for Israel to be "wiped off the map" but many analysts do not anticipate him seeking open military confrontation. They say that if Tehran does want a bomb, it is primarily for power-projection in the face of the U.S. "regime-change" campaigns in the region.

Even in Israel, some experts argue that a nuclear-armed Iran would likely be felt indirectly -- in emboldened support for armed groups like Lebanese Hezbollah or Palestinian Hamas, or occasional sabre-rattling aimed at denting the Jewish state's economy by scaring away investors and immigrants.

Israel's air force is training for long-range strikes, but Iran's nuclear sites may be too distant, numerous and fortified for it to tackle alone. After Hezbollah rocket salvoes during last year's Lebanon war, Israel is also in no rush to trigger Iranian retaliatory missile attacks on its home front.

And while there is speculation Israel's U.S. ally could take the lead in any future military action against Iran, the Bush administration may be overly mired in Iraq to trigger a new war.

Israeli Defence Minister Ehud Barak, during a meeting with his U.S. counterpart Robert Gates last month, voiced a circumspect view on Iran's atomic ambitions.

"There is already a Muslim bomb," an Israeli source familiar with the talks quoted Barak as saying. By referring to Pakistan, he was apparently playing down the potential destabilising impact if Iran one day joins the nuclear club.

Barak has championed Israel's development of a ballistic defence system to fend off any future Iran nuclear strike.

Israel is also building up a fleet of German-made submarines which are believed to carry nuclear missiles, a message that any catastrophic Iranian attack would be repaid in kind.

(Editing by Dominic Evans)