The very existence of the technology, known as a “two-point implosion” device, is officially secret in both the US and Britain, but according to previously unpublished documentation in a dossier compiled by the International Atomic Energy Agency (IAEA), Iranian scientists may have tested high-explosive components of the design. The development was today described by nuclear experts as “breathtaking” and has added urgency to the effort to find a diplomatic solution to the Iranian nuclear crisis.

The sophisticated technology, once mastered, allows for the production of smaller and simpler warheads than older models. It reduces the diameter of a warhead and makes it easier to put a nuclear warhead on a missile.