As the demonstrations have swelled in the streets of Tehran, the US and Britain have faced some difficult choices.

They don’t want to give the regime a pretext for blaming foreign powers for the turmoil, as it is determined to do. Convinced that there are overwhelmingly strong reasons to think the election result was rigged, they want to support any chance of a challenge to it, from the street or from senior politicians or clerics. But, expecting the result to stand anyway, they need to find some way of dealing with President Ahmadinejad.

Above all, Britain and the US need to work out quickly whether the Iranian leadership is prepared to negotiate about checking its nuclear programme.

Relations could be worse. Britain and the US have tried to stay out of what is internal Iranian turmoil. True, Iranian officials have twice called in the British Ambassador in Tehran (partly to complain about the coverage of the BBC’s new Persian service, which they have now shut down). Iran has also called in the Ambassador of Switzerland (which represents US interests) to protest about President Obama’s comments. But the British Government, for its part, has kept criticism to short, sharp comments by Gordon Brown and David Miliband.

Aside from those exchanges, outside governments sit and wait. There was genuine surprise at the declaration of a 63 per cent win for Ahmadinejad, because of the strength of support for Mir Hossein Mousavi in the days before the election, and a perception that the Supreme Leader and others might be seriously considering supporting that popular mood. They didn’t.

However, neither Britain nor the US — the two countries which have most actively tried to engage Iran and persuade it to drop its controversial nuclear work — wants to wait too long. Dr Mohamed ElBaradei, the Director-General of the International Atomic Energy Agency, the UN watchdog, said this week (to the BBC) that it was his “gut feeling” that Iran’s leaders wanted to put nuclear weapons within their reach “to send a message to their neighbours, to the rest of the world: don’t mess with us”. Iran claims that its uranium enrichment is to make fuel for power stations, but others fear that this conceals a weapons programme — and that Iran could reach that goal within a few years.

ElBaradei added that he thought Iran saw nuclear weapons technology as a means “to get that recognition . . . as a major power in the Middle East. It is also an insurance policy against what they have heard in the past about regime change” — meaning constant suspicion that the US or Britain will try to topple the regime.

US officials are emphatic that Obama’s strategy of trying to engage Iran, extending a hand of friendship, has not failed just because Ahmadinejad is still President. They will continue to make that offer, for now. Obama has already told Iran that he would reconsider the success of that offer at the end of the year, and that remains the plan.

European Union ministers meeting this week are also privately weighing their appetite for toughening sanctions on Iran, the next move if Ahmadinejad rejects the Obama offer. But the clock is ticking. Estimates vary as to how long it would take Iran to get a nuclear weapon — but that time is quickly passing.

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