The technical document describes the use of a neutron source, uranium deuteride, which independent experts confirm has no possible civilian or military use other than in a nuclear weapon. Uranium deuteride is the material used in Pakistan’s bomb, from where Iran obtained its blueprint.

“Although Iran might claim that this work is for civil purposes, there is no civil application,” said David Albright, a physicist and president of the Institute for Science and International Security in Washington, which has analysed hundreds of pages of documents related to the Iranian programme. “This is a very strong indicator of weapons work.”

The documents have been seen by intelligence agencies from several Western countries, including Britain. A senior source at the International Atomic Energy Agency (IAEA) confirmed that they had been passed to the UN’s nuclear watchdog.

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