Iran has enriched sufficient uranium to amass a nuclear bomb – a third more than previously thought – the United Nations announced yesterday.

The International Atomic Energy Agency said it had discovered an additional 209 kilograms of low-enriched uranium previously undeclared to inspectors, taking Tehran over the threshold of “nuclear breakout capability” of a ton of fissile material.

UN officials cautioned that many practical obstacles to the production of a bomb remain, and inspectors reported that Iran had slowed down on building its uranium enrichment capacity, a possible goodwill gesture to the new administration in Washington.

But the crossing of what has long been held as a red line in Iran’s nuclear programme dramatically add to the urgency in dealing with its ambitions and heighten tensions with Israel which has repeatedly declared it will not tolerate Iran reaching nuclear capability.

The additional uranium was discovered during its annual physical inventory of nuclear materials at the sprawling desert enrichment plant at Natanz.

Independent experts expressed astonishment at the mistake, which resulted in a miscalculation of the existing amount of uranium to which this year’s total was added.

Others voiced criticism that the inspections were only being carried out annually, given the urgency of the Iranian threat.

One expert, David Albright, a former UN nuclear inspector and the president of the Washington-based Institute for Science and International Security, declared himself “blindsided” by the report.

"They have reached a nuclear weapons breakout capability,” he said. “You can dance about it, but they would have enough to make 20-25 kg of weapons-grade HEU," or highly-enriched uranium, the final stage in refining fissile material for a nuclear bomb.

Experts had calculated that Iran was on track to meet this goal in six months to a year’s time, but that was before the miscalculation of stocks at Natanz was discovered.

The discrepancy came to light when the report noted that the new total came from the addition of 171 kilograms of new production to 839 kilograms of old production. But the agency had previously reported the old production as 630 kilograms – discrepancy of a third.

Officials said that Iran would be unable to enrich the uranium without using a separate clandestine facility.

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