The long-awaited report, released by the International Atomic Energy Agency, represents the strongest judgment the agency has issued in its decade-long struggle to pierce the secrecy surrounding the Iranian programme. The findings, drawn from evidence of far greater scope and depth than the agency has previously made public, have already rekindled a debate among the western allies and Israel about whether increased diplomatic pressure, sanctions, sabotage or military action could stop Iran’s programme.

Knowing that their findings would be compared with the flawed Iraq intelligence that preceded the 2003 invasion – and has complicated US moves on Iran – the inspectors devoted a section of the report to ‘credibility of information.’ The information was from more than 10 countries and from independent sources, they said; some was backed by interviews with foreigners who had helped Iran.

The report laid out the case that Iran had moved far beyond the blackboard to create computer models of nuclear explosions in 2008 and 2009 and conducted experiments on nuclear triggers. It said the simulations focused on how shock waves from conventional explosives could compress the spherical fuel at the core of a nuclear device, which starts the chain reaction that ends in nuclear explosion.

The report also said Iran went beyond such theoretical studies to build a large containment vessel at its Parchin military base, starting in 2000, for testing the feasibility of such explosive compression. It called such tests “strong indicators of possible weapon development.”

The inspectors agreed with a much-debated classified US National Intelligence Estimate issued in 2007 that Iran had dismantled a highly focused effort to build a bomb in late 2003 but found significant recent work, although conducted in a less coordinated manner.

The report does not claim that Iran has mastered all the necessary technologies, or estimate how long it would take for Iran to be able to produce a nuclear weapon. Inspectors do not point to a single weapons lab, or provide evidence of a fully constructed nuclear weapon.

Instead, the report describes roughly a dozen different projects that countries that have built nuclear weapons – the United States, Russia, China, Britain, France, Israel, India and Pakistan – all had to grapple with, in some form. An IAEA report in May listed five fewer categories of such technical information.

* Documents suggesting that Iran “was working on a project to secure a source of uranium suitable for use in an undisclosed enrichment programme” to make bomb fuel.

* Evidence that Iran “had been provided with nuclear explosive design information.”

* Information that it worked on experiments with conventional explosives to compress metal into an incredibly dense mass suitable to start a chain reaction.

* Documentation of “at least 14 progressive design iterations” for a missile warhead to deliver an atomic warhead to a distant target.

The report was produced under Yukiya Amano, a former Japanese diplomat who has run the IAEA for nearly two years, and addressed to the agency’s board of governors and the UN Security Council. In it, Amano said that inspectors had amassed ‘over a thousand pages’ of documents, presumably leaked out of Iran. He said they showed “research, development and testing activities” on technologies that would be useful in designing a nuclear weapon.

Iran quickly rejected the report’s findings. “The report of the International Atomic Energy Agency is unbalanced, unprofessional and politically motivated,” Ali Asghar Soltanieh, Iran’s representative to the IAEA, was quoted as saying by the country’s official Islamic Republic News Agency.

President Mahmoud Ahmadinejad of Iran said the UN agency should instead investigate the US nuclear arsenal. “If the agency is after the truth, why has it not released any report on the US atomic bombs concealed in 1,000 of its military bases?” Ahmadinejad was quoted as saying by the semiofficial Fars news agency.

Iranian officials have said the evidence is fabricated, and some have warned that any attempt by the west to stop its programme, by any means, could invite retaliation.

Amano said the agency had “tried without success to engage Iran in discussions about the information.” But he said that “Iran continued to conceal nuclear activities,” including its effort to construct a secret enrichment facility near Qum.

Iran told the nuclear agency about that facility days before President Barack Obama and European leaders reported its existence two years ago, and Iran has recently said it is moving some of its nuclear activity to that underground facility, at a well-defended military base.

Role reversal

The IAEA report’s detailed revelations are a fascinating role reversal from 2003, when the US and Britain claimed Iraq was seeking to rekindle its nuclear programme. In that case, the agency warned that the Bush administration’s case was weak and that some of the evidence was forged. Now, it is the normally cautious agency that is taking the lead, arguing that years of study had led it to the conclusion that, despite Iran’s denials, the country engaged in an active programme to design nuclear warheads, among other technologies.

A western diplomat familiar with the agency’s report said it was “much more detailed and far more confident than anything they have done before.”

“The level of detail is unbelievable,” said a western diplomat familiar with the report, who spoke on the condition of anonymity because he was discussing the agency’s internal assessment of the report. “The report describes virtually all the steps to make a nuclear warhead and the progress Iran has achieved in each of those steps. It reads like a menu.”

The new evidence came as no surprise to the Obama administration, which has possessed some of this intelligence for years but moved carefully in adding pressure on Iran, mindful of the loss of credibility the US suffered over faulty intelligence on Iraq’s weapons efforts.

Now the US faces a new set of difficult choices. Years of sanctions have hurt Iran, but the report makes clear that those sanctions have not forced it to reconsider its programme.

Efforts to sabotage the Iranian effort have reached back a decade, most recently with a computer worm called Stuxnet, which appears to have been a joint covert action by Israel and the US. It is not mentioned in the report, but experts say it slowed Iran’s enrichment of uranium. But production rates have since recovered.

While Israel has talked about military action, both the Bush and Obama administrations have argued that an airstrike would not slow the programme much, and that it would drive it further underground. George W Bush denied Israel some of the technology it requested to make a strike more effective. But there are many theories about whether Israel’s latest discussions of military strikes are intended to focus the west on new pressure and sanctions, or are leading up to military action.