The British, French and German foreign ministers and Javier Solana, the EU foreign policy chief, jointly wrote a letter warning of dire consequences if Tehran breaks a commitment made last year to halt the conversion of uranium ore at its nuclear site in Isfahan.

The showdown threatened to trigger a diplomatic row on the eve of the inauguration today of Mahmoud Ahmadinejad as the Iranian President.

“Were Iran to resume currently suspended activities, our negotiations would be brought to an end, and we would have no option but to pursue other courses of action,” the letter said. It was signed by Señor Solana, Jack Straw, the Foreign Secretary, and his French and German counterparts, Philippe Douste-Blazy and Joschka Fischer. “We therefore call upon Iran not to resume suspended activities or take other unilateral steps.” The letter was addressed to Dr Hassan Rouhani, the head of the Iranian National Security Council.

Despite fears in the West and the Middle East about Iran acquiring a nuclear capability, there are deep divisions among intelligence analysts over how long it would take the Islamic state to build an atomic bomb.

According to a new US National Intelligence estimate, Iran is still a decade away from producing enough highly enriched uranium to begin production of nuclear weapons.

The assessment, which was reported in The Washington Post yesterday, contradicted earlier predictions that put Iran five years away from building an atomic bomb. Only last week Silvan Shalom, the Israeli Foreign Minister, issued a warning that Iran was only months away from the technology needed to unlock the secrets of how to turn fissile material into weapons. Israeli officials have since estimated that it would take Iran until 2008 to build a nuclear bomb.

The Iranians insist that they simply want to build a civilian nuclear programme for power generation, as they are permitted to as members of the International Atomic Energy Agency and signatories of the nuclear non-proliferation treaty. But they have raised suspicions by pursuing much of the work in secret and by their determination to build a fuel cycle that could have military as well as civilian applications.

Yesterday the Iranians made clear their intention unilaterally to scrap an agreement reached in Paris last year to suspend their controversial work. The process includes converting and enriching uranium, which provides the core of an atomic bomb.

The EU foreign ministers had hoped by August 7 to propose a package of concessions that would allow Iran to operate a civilian nuclear programme but prevent it from acquiring weapons-grade material. But British officials said that the offer would be “null and void” if the Iranians went back on their commitments.

Certainly Iranian leaders suggested yesterday that the EU initiative could come too late. Mohammad Khatami, the outgoing reformist President, said: “The decision has been taken. Work has begun.”

If the Iranians do resume work on converting and enriching uranium, the matter will almost certainly be referred to an emergency meeting of the IAEA’s board, which could be convened at its headquarters in Vienna in as little as 72 hours.

Britain and the other EU members want to keep international consensus and would probably agree to one final warning to Tehran before the matter is referred to the UN Security Council.

At that point America, Britain and France could press for sanctions to be imposed against the Iranian regime unless it abandons its nuclear ambitions.

But the Iranians may have calculated that they can drag the process out by splitting the IAEA and blocking UN action, thanks to their strong relations with Russia and China, the other two permanent members of the UN Security Council.

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