Iran could develop a nuclear weapon within a year, a leading think-tank has warned.
Under the most likely scenario it would take Tehran over two years to make a single atomic bomb, according to the International Institute for Strategic Studies (IISS).
But if Iran was able to make untested uranium enrichment methods work, this timeline could be much shorter.
One remote possibility mapped out by the IISS would see Iranian scientists able to produce enough highly-enriched uranium for a weapon in just four weeks.
Tehran insists its nuclear programme is for peaceful civil purposes, but there are growing concerns in the West that it is being used to mask attempts to develop a bomb. Defence Secretary Liam Fox warned in the Commons on Monday that it was "entirely possible" the Islamic Republic might have developed a nuclear weapon by next year.
The evidence shows "beyond reasonable doubt" that Tehran is seeking the ability to produce nuclear weapons, the London-based IISS concluded in a report on Iran's nuclear, chemical and biological capabilities. Iran's current stocks of low-enriched uranium would be enough for one or two nuclear weapons if further enriched, the study found.
At the report's launch, Sir Richard Dalton, a former British ambassador to Tehran, asked about the possibility of Iran using more advanced "third generation" centrifuges to enrich uranium for a weapons programme.
Lead author Mark Fitzpatrick, director of the IISS non-proliferation and disarmament programme, replied: "That's the scary part. If Iran were able to get the third generation working well and producing in large numbers, the time to produce a weapon's worth of HEU (highly-enriched uranium) reduces to four weeks.
"If they had a clandestine plant with almost 6,000 centrifuges operating - the number that were in the AQ Khan design that they sold to Libya - if they had 6,000 of these working and they were this most advanced third generation, four weeks."
Mr Fitzpatrick stressed that this was only a theoretical scenario. The IISS also concluded that claims about Iran's supposed chemical and biological weapons programmes could not be confirmed from publicly-available information and "may have been exaggerated".