The Foreign Secretary described the idea that the White House wanted a nuclear strike as “completely nuts”. He insisted that Britain would not support pre-emptive military action, adding: “I’m as certain as I can be sitting here that neither would the United States.”

Many analysts in the West suspect that Tehran is attempting to build its own nuclear weapons. Over the weekend Iran allowed UN inspectors to examine some of the atomic plants which, its Government maintains, are designed solely for production of electricity.

Speaking to the BBC, Mr Straw said: “There is no smoking gun, there is no casus belli. We can’t be certain about Iran’s intentions and that is, therefore, not a basis on which anybody would gain authority to go for military action.”

He was responding to a new article by Seymour Hersh, published in The New Yorker. It has been seized on as evidence that any hope of a diplomatic solution to the stand-off is being swept aside by White House hawks.

Hersh says that President Bush now believes his historic purpose is to stop President Ahmadinejad, whom he is said to regard as a “new Hitler”, acquiring nuclear weapons.

The article suggests that Pentagon plans presented to the White House include the use of a “bunker-buster” tactical nuclear weapon, such as the B61-11, against underground sites in Iran because of concerns that conventional strikes would not be “decisive”. Hersh says that some senior military officers are so alarmed about Mr Bush’s willingness to use nuclear weapons that they are ready to resign in protest.

Yesterday the Pentagon attempted to dismiss the report as being filled with “fantastical, wrong and unsubstantiated allegations”. Hersh pointed out that the Pentagon had used similar language initially to describe his revelations about abuses at Abu Ghraib prison in Baghdad. His article states that American combat troops have been ordered to infiltrate Iran to collect target data and to cultivate relationships with indigenous groups that oppose Tehran. It also claims that US carrier attack jets have been flying simulated bombing runs within range of Iranian coastal radar.Pentagon officials denied this yesterday. They said that war planners have routinely updated contingencies for dealing with Iran’s nuclear ambitions but this did not reflect any orders to prepare for a military confrontation.

The US is thought to have taken limited steps that go beyond contingency planning, such as flying drones over Iran.

The Iranian Foreign Ministry dismissed the US media reports yesterday as a form of “psychological warfare” that stemmed “from America’s anger and helplessness”.

Hamid Reza Asefi, its spokesman, also reiterated that the presence of UN inspectors showed that Iran was willing to co-operate and that its intentions were peaceful.

Last month the UN Security Council gave Iran 30 days to halt its nuclear research, or risk action such as sanctions.

Joseph Cirincione, of the Carnegie Endowment for International Peace, said: “I previously dismissed talk about US military strikes as left-wing conspiracy theory . . . But in just the past few weeks I’ve been convinced that at least some in the Administration have already made up their minds that they would like to launch a military strike against Iran.”

Mr Straw, who has described such a scenario as inconceivable, acknowledged yesterday that the US Administration uses “slightly different language” on the issue. “President Bush says (military action) is not on the agenda, but they don’t rule out any option in theory. I believe it is not on the agenda and they are very committed indeed to resolving this issue by negotiation.”

Mr Straw said that he was encouraged that Russia and China had joined the US and EU powers to apply diplomatic pressure on Tehran.

Kori Schake, a former staffer on the US National Security Council, was quoted by The Washington Post as calling renewed talk of a military strike a “diplomatic gambit to keep pressure on others”.

FOUR YEARS OF DEADLOCK

• Iran began work on a nuclear reactor in 2002 with the help of Russian engineers
• US satellite images showed nuclear sites under construction at Natanz and Arak the same year
• A report by the UN International Atomic Energy Agency in 2003 found that Iran had been concealing a uranium enrichment programme for 18 years
• Iran promised to suspend the programme in November 2004 after talks with Britain, France and Germany
• The US has taken a tougher approach, urging sanctions and refusing to rule out military force
• Uranium enrichment resumed in August last year after Mahmoud Ahmadinejad, a hardliner, was elected President of Iran
• In February the IAEA reported Iran to the UN Security Council
• The Security Council has given Iran a 30-day ultimatum to suspend uranium enrichment, expiring at the end of the month

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