“Additional time - up to two years, according to conservative estimates - would be required for Iran to build a nuclear warhead that would be reliably deliverable by a missile,” they said in a report published last month.

Mark Fitzpatrick, of the International Institute for Strategic Studies (IISS) think-tank, also said Iran would need at least two years for assembling a nuclear-tipped missile.

Senior researcher Greg Jones of the U.S.-based Nonproliferation Policy Education Center put forward a much quicker breakout scenario for any bomb bid and suggested a truck rather than a missile could be used for delivery to target.

Iran could refine uranium for a nuclear weapon in 10 weeks and produce the required non-nuclear components in six months or less, he said, adding this could be done simultaneously.

NO BREAKOUT WITH JUST ONE BOMB?

But the IISS argued in a report last year that the weaponisation time must be added to that required to produce the fissile material to calculate when a usable bomb could be made.

Making the actual weapon entails converting uranium gas to metal, designing a nuclear triggering device and the production and fitting of spherical explosive lenses, it said.

The United Nations’ International Atomic Energy Agency (IAEA) last year published a report with a trove of intelligence indicating past, and some possibly continuing, research activities in Iran that could be relevant for nuclear weapons.

They included suspected high explosive experiments and possible work on designing a device to produce a burst of neutrons for setting off a fission chain reaction.