Both Bush and Dick Cheney, his vice-president, regard Mahmoud Ahmadinejad, Iran’s president, as a new Hitler who cannot be allowed to acquire nuclear weapons and carry out his fantasy of wiping Israel off the map.

Although they hope that diplomatic efforts to restrain Iran will succeed, “it is not in their nature to bequeath the problem to their successors”, a senior White House source said last week.

The Pentagon is believed to be considering options that would allow it to destroy facilities such as Iran’s main centrifuge plant at Natanz in a single night of bombing.

Richard Perle, a leading neoconservative, said that an attack could “be over before anybody knew what had happened. The only question then would be what the Iranians might do in retaliation”.

Defence analysts believe the most likely weapon is Big Blu, a 30,000lb bunker-buster bomb that will be ready for use towards the end of 2007.

A report by Seymour Hersh, the investigative reporter, in tomorrow’s New Yorker magazine claims the Pentagon is also considering the use of a tactical nuclear weapon. A refusal to rule out the nuclear option has reportedly led some officers to talk of resigning.

“There are very strong sentiments within the military against brandishing nuclear weapons against other countries,” Hersh quotes a Pentagon adviser as saying.

The Bush government has been inviting defence consultants and Middle East experts to the White House and Pentagon for advice.

The favoured scenario is an attack using a small number of ground attack aircraft flying out of the British dependency of Diego Garcia in the Indian Ocean. The British would have to approve the use of the American base there for an attack and would be asked to play a supporting role by providing air-to-air refuelling or sending surveillance aircraft, ships and submarines.

Senior Pentagon planners recently advised the White House that they did not yet have accurate intelligence on the whereabouts of all Iran’s nuclear enrichment sites and several were buried under granite. At present it could hope to set back the Iranians’ nuclear programme by only two years.

American officials remain divided about the wisdom of a military strike. A senior White House source said opinion was in a “state of flux” and added: “We can bomb the sites, but what then?” It was important to plan for an escalation of the conflict, the source said.

The assumption that British forces would take part in an attack on Iran will be deeply embarrassing to the government. The Foreign Office has insisted that a diplomatic solution is still possible.

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