Norman Dombey and Claire Spencer: Whether or not they've been looking for a pretext to bomb Iran, they've found one

If Iran was an 'imminent threat', that might authorise an attack

"The United States should immediately launch a new diplomatic offensive to build an international consensus for stability in Iraq and the region.

"This diplomatic effort should include every country that has an interest in avoiding a chaotic Iraq, including all of Iraq's neighbours."

This was the main recommendation of the bipartisan Iraq Study Group, headed by the former US Secretary of State James Baker and the former Democratic Congressman Lee Hamilton, announcing its conclusions two months ago. It also recommended a change in the primary mission of US forces in Iraq that would enable the US to begin to move its combat forces out of Iraq, and called for action to bring an end to the Israeli-Palestine conflict.

Was this to be the signal that US policy was about to change? Not a bit of it. In response, President Bush announced, instead, a "surge" of 20,000 new US forces to go to Baghdad to take on the Shia militias. He also refused to consider talks with Iran and Syria.

So where does Tony Blair stand? He welcomed the Baker-Hamilton report, but also managed to back the troop surge. In its approach to Iran's purported interest in acquiring nuclear weapons UK policy used to beto search for a diplomatic solution - and EU partners and the Study Group report took a line consistent with that approach. Yet poor Jack Straw lost his job as Foreign Secretary soon after saying that military action against Iran was inconceivable.

Since late 2006, Blair has ventilated his concerns (apparently inspired by the Bush administration) about Iran's export of terrorism and its meddling in Iraq. In December, he said that the world needs to "wake up" to the "major strategic threat" posed by an Iran that was "openly supporting terrorism in Iraq", thus again allying himself with Bush's rejection of the report.

Blair seems not to want to rule anything out. Already there are concerns that, following the surge in US troop numbers, Shia forces in and around Basra will retaliate against British troops this spring, just when the plan is to draw down 3,000 of the remaining 7,000 UK forces in southern Iraq. Yet that would be nothing compared with the dangers of retaliation if forecasts of a US strike on Iran are accurate.

Even if the UK were to distance itself from US military action against Iran - as Blair did when the idea was first raised last April - the long-standing identification of the British with US policy in the region would make UK forces an attractive "soft-target" for any Iranian Revolutionary Guards or their Iraqi proxies operating in southern Iraq.

As the tension increases, what prospects are there for a diplomatic solution? The UN Security Council has suspended Iran's right to have any nuclear capability other than the electricity-generating reactor at Bushehr (which Russia is building). It also imposed limited sanctions on individuals and organisations connected with Iran's nuclear and missile programme. Further, it set a 60-day limit for the International Atomic Energy Agency (IAEA) to report back on Iranian compliance. Since the one foreign policy issue to have unerringly united the internally fractious Iranians is resistance to suspending anything under external pressure, this avenue does not look fruitful. This week's appeals by the head of the IAEA, Mohammed ElBaradei, for a "time out" on both enrichment and international sanctions seem likely to fall on deaf ears.

Few American or European diplomats are prepared to recognise that their insistence on imposing pre-conditions for a return to negotiations has consistently empowered the hardliners in the Iranian government.

The diplomatic route is full of pitfalls. The problems for Washington's hawks, though, is that they have to seek authorisation for military action under international law. If, however, Iran were deemed to present an "imminent threat to international security", that would be a different matter. This may explain why we have heard so much recently about Iran's nuclear intentions. But the facts don't support the hawks.

At present, Iran has installed 328 old-fashioned uranium centrifuges that are copies of a Dutch design from 30 years ago. It has successfully enriched uranium to five per cent but not produced more than a few grams of enriched material. To make a nuclear weapon it would need to make about 40kg of uranium enriched to 90 per cent. Even with the 3,000 centrifuges that President Ahmadinejad claims are planned, it would probably take about two years to install and run them and another two before they could enrich enough uranium for one weapon. And the enrichment plant at Natanz is under continuous IAEA camera surveillance. So there is no imminent nuclear threat; there is plenty of time for a negotiated solution if the US wished to find one.

So, whether the State Department likes it or not, Iran is some way off having the bomb. Which may be why a new tack has emerged. Official US declarations, backed by Blair, maintain that Iranian involvement in Iraq constitutes a level of external interference that the US will not tolerate indefinitely. In the immediate aftermath of Bush's announcement of the troops surge in Iraq, no fewer than three public declarations were made to this effect: by Bush, by Secretary of State Condoleezza Rice, and by Vice-President Dick Cheney. The stakes were raised last month by the detention by US forces of seven Iranians in Iraq. They have since been ratcheted higher by the "shoot-to-kill" policy against Iranians in Iraq declared recently. Last Monday, Bush ominously declared that "if Iran escalates its military actions in Iraq", the US "will respond firmly".

If the US has been looking for a pretext to bomb Iran, it seems to have found one: that Iran poses a threat to Iraqi sovereignty. Since air-strikes against nuclear targets were mooted last summer, their military feasibility and diplomatic advisability have been questioned. If the rationale for these attacks were shifted away from nuclear targets, the US might follow Israel's attack on Lebanon last year and aim to take out a wide range of Iranian assets. The New Yorker's Seymour Hersh claimsat least 400 targets had been identified.

The upsurge in official US statements indicates that the Pentagon's contingency planning for Iran is more than a distant "worst-case" scenario. One rumour gaining currency is that Bush wants to attack Iran this spring - before Blair resigns, to be able to rely on his support. It would be remarkable if Blair, now under severe pressure at home, were to have a last fling with Bush.

Gordon Brown might wish to consider the consequences for British interests if air-strikes against Iran were to go ahead before he takes office.

Claire Spencer is head of Chatham House's Middle East programme. Norman Dombey is Professor Emeritus of Theoretical Physics at Sussex University