A report by the UN's nuclear watchdog due to be circulated around the world next week will provide fresh evidence of a possible Iranian nuclear weapons programme, bringing the Middle East a step closer to a devastating new conflict, say diplomats.

The report by the International Atomic Energy Agency (IAEA) is the latest of a series of quarterly bulletins on Iran's activities, but this one will contain an unprecedented level of detail on research and experiments carried out in Iran in recent years, which western officials allege could only be for the design and development of a nuclear warhead. "This will be a game-changer in the Iranian nuclear dossier," a western official predicted. "It is going to be hard for even Moscow or Beijing to downplay its significance."

The key passage of the "safeguards report" will be a summary of all the evidence collected over the years by UN weapons inspectors, including a substantial amount of hitherto unpublished data pointing to work in the past seven years.

Western officials say Iranian work up to 2003 involved research and engineering, including the production of some prototype components of a warhead. From 2004, alarmed by the invasion of neighbouring Iraq, those officials say Iranian technicians pursued only design work and computer modelling to reduce the chances of being detected.

Iran has consistently stated that its nuclear programme is for peaceful means. In the report to be circulated among IAEA member states, probably on Wednesday or Thursday of next week, the agency's director-general, Yukiya Amano, is not expected to draw definitive conclusions, as the US, Britain and France had hoped. But his inspectors will draw attention to experimentation with few, if any, applications outside nuclear weaponry.

Some of the evidence has been supplied by US, British, Israeli and other western intelligence agencies, and those agencies are believed to have vetted it for publication, but diplomats say it will be cited only where IAEA experts have been able to corroborate the information independently.

The report will almost certainly raise tensions in a region made volatile by this year's Arab revolutions and the turmoil in Syria. In the absence of a tough new UN security council resolution, the US will face the dilemma of acting militarily without an international mandate, or risk missing Iran's window of vulnerability to attack.

How fast that window closes will be determined by the progress of Iran's nuclear programme. Western experts believe that if Iran decides to break out of international constraints and race to make a bomb, its technicians will take just months to solve the problems of fabricating a small warhead for missile delivery. The biggest challenge is making the fissile material to put inside it.

Weapons-grade uranium requires more than a 90% concentration of the element's most fissile isotope, U235. Most of Iran's stockpile is low-enriched uranium (LEU), with a 3.5% concentration, made by hundreds of high-speed centrifuges spinning at a plant at Natanz, central Iran, in defiance of UN security council resolutions. According to the last IAEA report, issued in September, Iran has amassed 4.5 metric tonnes of LEU, enough if further enriched to weapons grade to make three to four warheads.

Enriching to 90% is not easy, as the level of impurities in the uranium fuel becomes more of a challenge. However, since February 2010 Iran has been successfully making 20%-enriched uranium at Natanz, ostensibly to fuel a medical research reactor in Tehran.

Western governments allege this is a pretext as Iran lacks the means to manufacture the necessary fuel rods. They point out that, in terms of technical difficulty, 20% uranium is nine-tenths of the way to weapon-grade material. In fact, leaked US diplomatic cables reveal that as far back as April 2009 US officials were convinced that Iran had mastered the process.

Iran has more than 70kg of 20% uranium – about a fifth of the quantity needed to make a bomb if further enriched. Of even greater international concern was the confirmation in the September IAEA report that Iran had installed a set, or "cascade", of centrifuges at a new site at Fordow, near to the Shia holy city of Qom.

The Fordow site, whose existence was revealed in 2009, is under a mountain and would be extremely difficult to damage by aerial bombing. Iranian authorities claim 10 other enrichment sites are being prepared but no sign of them has materialised.

At the moment it is the transfer of enrichment to Fordow that represents the ticking clock for western military intervention. Once the bulk of production is established there, the programme would be a much harder nut to crack.

The transfer of Iran's stockpile of 20% uranium from the relative vulnerability of Natanz to the impregnability of Fordow would be seen as even more threatening. "That would be a huge red line – a very significant move that would be very hard to ignore," a western diplomat said.

The independent Institute for Science and International Security estimates that if Tehran took the decision to make a weapon it would take about six months for Iran to "break out" and make enough weapons-grade uranium for a single warhead. It would take three years to build a modest arsenal, less if Iran succeeds in perfecting a new generation of centrifuges with carbon-fibre and specialised steel parts in place of aluminium, but international restrictions on those materials appear to be slowing that effort.

The performance on the original aluminium IR-1 centrifuges also seems to be declining from IAEA figures, either because of wear and tear or because of sabotage like the Stuxnet computer worm. However, the Iranians have surmounted the problem

by using more and more IR-1 and the stockpiles of enriched uranium have mounted slowly but relentlessly. Stuxnet appears to have been, at most, a hiccup for Iran's nuclear ambitions.

Military force would be a heavier but blunter tool, and its efficacy could never be assured. No one outside the Iranian regime can be sure whether there is a covert, parallel programme mirroring what can be seen from the air, of which the mountain at Fordow is just the tip of a nuclear iceberg.