An Israeli colleague was sent on an assignment so secret and sensitive that it was years before he would share the full story with friends.

He was dispatched by Menachem Begin, then the Prime Minister, to European capitals with orders to meet editors, politicians and opinion makers to spread the word that Israel was increasingly concerned about Iraq's nuclear programme and would do anything to stop Saddam Hussein building the bomb. The warnings, intended to prepare Western public opinion, were largely dismissed as sabre-rattling (one editor insisted on discussing a new lavatory system designed on a kibbutz) - until June 1981, when Israeli Air Force F16s bombed the plant to rubble.

A few days ago a chill went down my spine when an articulate and intelligent senior Israeli official made exactly the same argument about Iran's nuclear programme at a briefing in London. He described an Iranian nuclear weapon as an existential threat to the Jewish state, which would defend itself whatever the consequences. These warnings are not new but the political and military circumstances are conspiring to make an Israeli attack on Iran a probability, unless the Middle East experiences dramatic changes in the coming weeks and months.

It is a widely held conclusion among nuclear experts that Iran now possesses enough enriched uranium to build a nuclear bomb. It would still have to be enriched to weapons grade at the centre in Natanz before being made into a warhead. But Iran has mastered the technology and has the raw materials. Building a nuclear bomb is now only a matter of time.

Iran's presidential elections are in June. President Ahmadinejad is expected to be re-elected. Indeed, a Western diplomat in Tehran said that he had not met a single Iranian - even opponents of the Government - who did not believe that he would be returned with a healthy majority. He has vowed repeatedly to press ahead with Iran's nuclear programme and appears to have the full support of Ali Khamenei, Iran's Supreme Leader.

The combination of the two events is seen in Israel as crossing a red line. Mr Ahmadinejad has threatened to wipe Israel off the map, has hosted a Holocaust-denying conference, and has stepped up arming and funding Hezbollah and Hamas, the two militant groups responsible for rocket attacks against Israel. If he is re-elected for another term with the prospect of building a bomb, Israel would do anything to stop him.

This bleak outlook is made even more sombre by the formation this week of a new Israeli Government under the leadership of Binyamin Netanyahu with Ehud Barak, the Labour leader and junior coalition partner, as the Defence Minister. What is significant is not their political affiliations but their military background. Mr Barak, the most decorated soldier in the Israeli army, once headed Sayeret Matkal, Israel's equivalent of the SAS before becoming the army chief. One soldier serving under him was Mr Netanyahu. Another veteran of this elite unit was Moshe Yaalon, also in the Cabinet. These men have taken part in assassination operations against Palestinian leaders and commanded daring raids deep inside enemy territory. In short, they have the experience and the confidence to plan and execute an attack on Iran.

Indeed, Mr Barak was Defence Minister in the previous Government when Israel carried out its latest secret raid in January - on a weapons convoy in Sudan. According to details released this week, Israeli F16 bombers, protected by F15 fighters, attacked targets in Sudan. Pilotless drones then filmed the wreckage, relaying back images which revealed that some vehicles were undamaged. The jets then flew a second sortie. The aircraft, which were refuelled in mid-air, flew 1,750 miles from Israel to Sudan and back. The distance from Israel to Natanz, the uranium enrichment centre in Iran, is 900 miles one way.

A factor in any Israeli calculation will be Iran's air defences, which are far more daunting than Sudan's. Here too there is good reason to believe that Israel may act sooner rather than later. Russia has sold Iran the sophisticated S300 surface-to-air system. Israel would want to launch an attack before these missiles are in place.

These military imperatives might make sense to soldiers, but surely the political cost of a pre-emptive raid - not to mention the risk of plunging the Middle East into another big war - would rule out an attack.

This argument might make sense from Europe but in the Middle East quite another logic is at work. Many Arab states, particularly in the Gulf, are more afraid of a nuclear-armed Iran than Israel is. A military strike that delayed that threat would be welcomed in some Arab capitals. The Israelis know that they would face a huge international outcry. But that happened after the raid on Iraq and many countries later thanked them privately. More recently they were widely attacked after the offensive against Gaza in January, but over time that criticism has died down.

Today the only serious obstacle to this battle is Barack Obama. He has launched a diplomatic offensive aimed at repairing ties with Iran and re-engaging with the regime after 30 years of hostility. There are some signs that Tehran is interested. An Iranian envoy attended a recent meeting on Afghanistan alongside a US delegation. Similar talks have also taken place on Iraq.

But these gestures are largely futile unless Washington can persuade Tehran that it is in its own best interests to shelve its nuclear programme, rejoin the community of nations and co-operate with America. That is a big step for a regime that came to power promising an Islamic revolution and continual struggle with America and Israel.

When Mr Netanyahu travels to Washington next month, Iran is expected to dominate talks. Israel will not attack Iran without tacit approval from America. But time is running out. This could become Mr Obama's biggest challenge.

Richard Beeston is foreign editor of The Times

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