Most member states of the International Atomic Energy Agency (IAEA), meeting in Vienna, are expected to vote to report Tehran to the UN Security Council.

Everyone from American hawks to Iranian militants insist that a negotiated solution is still possible. Under such a solution Iran would build a legitimate civilian nuclear programme without joining the exclusive club of nuclear-armed states.

Privately, experts predict a far darker scenario. They now believe that within five years Iran will either succeed in building an atomic bomb or be prevented by a US military attack that could trigger a broader Middle East conflict.

The apocalyptic predictions are far from fanciful, and sober voices freely admit to sleepless nights as they plot the next stages in the high stakes game of international politics.

If the IAEA reports Iran to the Security Council, President Ahmadinejad has made it clear that a referral would invite immediate retaliation. Tehran would cease co-operation with the IAEA, banning its inspectors from nuclear sites, which would resume full uranium enrichment work and whip up anti-Western forces across the the region.

For the next month the world’s top diplomats will talk Iran down from its position and accept a compromise, involving uranium enrichment work being conducted for Iran on Russian soil.

But if no deal is reached the matter will be taken before the Security Council, which has the power to impose sanctions and in extreme cases authorise the use of force.

While no one is yet pressing for sanctions, apart from the United States, which already has a trade embargo against Iran dating back a quarter of a century, Tehran’s defiant rhetoric and provocative actions could make the issue unavoidable.

As with the debate over Iraq’s weapons of mass destruction, the Security Council’s five permanent members are split.The United States, Britain and France might consider sanctions aimed at Iran’s nuclear industry, its military and political leadership. But Russia and China are hesitant. Russia has billions of pounds of contracts with Iran, including a deal to build a nuclear reactor and to provide air defence systems. China receives about 15 per cent of its oil from Iran.

If sanctions were imposed, Iran could use its navy and shore batteries to attack shipping from the Gulf passing through the Straits of Hormuz, a choke point for oil and gas exports from Iraq, Kuwait, Saudi Arabia and Qatar.

Iran could also provoke attacks against Western targets using its proxies in the region. It could mobilise Hezbollah, the Shia Muslim militia in Lebanon; Hamas, the Palestinian extremist movement that won parliamentary elections last month; the powerful al-Mahdi Army of Hojatoleslam Moqtada al-Sadr in Iraq; and smaller groups in Afghanistan.

Western analysts believe that during this war of attrition, Iran would attempt to accelerate its nuclear programme unhindered by outside interference. The country could master in about a year the technology to make fissile material, the core of a nuclear bomb. It would take another three years to assemble enough highly enriched uranium for a warhead.

Before that happens the United States, Israel and other allies may have concluded that force must be used. The United States is the only country capable of launching the operation, which would be likely to target a dozen nuclear sites, as well as medium-range missiles, air force bases and other facilities that could be used in a counter-attack.

In the worst case, military action could trigger a war across the Middle East. Alternatively, a country that has vowed to “wipe Israel off the map” would be a nuclear power. Turkey, Egypt and Saudi Arabia would inevitably race to build their own nuclear deterrents. A nuclear war in the region would become a real possibility. Either way the consequences could be disastrous.

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