The Pentagon position was described, by the committee chairman, Mike Gapes, as throwing a demand for a militarily enforced embargo into the security council “like a hand grenade – and see what happens”.

Yesterday Mr Bolton reiterated his hardline stance. In a speech to the annual convention of the American-Israel public affairs committee, the leading pro-Israel US lobbyists, he said: “The longer we wait to confront the threat Iran poses, the harder and more intractable it will become to solve … we must be prepared to rely on comprehensive solutions and use all the tools at our disposal to stop the threat that the Iranian regime poses.”

The IAEA referred Iran to the security council on February 4, but a month’s grace was left for diplomatic initiatives. By yesterday, those appeared exhausted. A meeting of European and Iranian negotiators broke down on Friday over Tehran’s insistence that even if Russia was allowed to enrich Iran’s uranium, Iran would enrich small amounts for research. Iran says that it needs enrichment for electricity.

According to Time magazine, the US plans to present the security council with evidence that Iran is designing a crude nuclear bomb, like the one dropped on Nagasaki in 1945. The evidence will be in the form of blueprints that the US said were found on a laptop belonging to an Iranian nuclear engineer, and obtained by the CIA in 2004. However, any such presentation will bring back memories of a similar briefing in February 2003 in which Colin Powell, then US secretary of state, laid out evidence of Iraqi weapons of mass destruction, which proved not to exist.

While the US and Britain keep a united front over Iraq in the UN security council, there are clear differences over Iran. Britain has ruled out a military option if diplomatic pressure fails. The US has not. There is no serious consideration of large-scale use of ground forces, but there are disagreements in the administration over whether air strikes and small-scale special forces operations could be effective in halting or slowing down Iran’s alleged nuclear weapons programme.

Some believe Iran has secret facilities that are buried so deep underground as to be impenetrable. They argue that the US could never be certain whether or not it had destroyed Iran’s “capability”.