The prospect of fresh tension between the UK and Iran arose today after a document emerged suggesting the Middle Eastern country was working on testing a key nuclear bomb part.
The Times newspaper said it had obtained notes describing a four-year plan to test a neutron initiator, the component of a nuclear bomb that triggers an explosion.
Several experts believe that the documents are the strongest indicator yet of a continuing nuclear weapons programme in Iran.
The technical document describes the use of a neutron source, uranium deuteride, the Times said, which is feared to have no possible civilian or military use other than in a nuclear weapon. Uranium deuteride is reportedly the material used in Pakistan's bomb, from where Iran obtained its blueprint.
Foreign intelligence agencies have apparently dated the document to early 2007, four years after Iran was thought to have suspended its weapons programme, and it is understood to have been passed on to the UN's nuclear watchdog.
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, told the Times: "Although Iran might claim that this work is for civil purposes, there is no civil application.
"This is a very strong indicator of weapons work."
A spokesman for the Foreign and Commonwealth Office said: "We do not comment on intelligence, but our concerns about Iran's nuclear programme are clear and based on information in the public domain."
Tension between the west and Tehran over its nuclear programme has been rising in recent weeks.
Last month Iran unveiled plans to build 10 new uranium enrichment plants in breach of United Nations resolutions.
President Mahmoud Ahmadinejad revealed he had ordered five sites to be built and locations found for five more.
The announcement came just two days after Tehran was censured by the international community over its existing nuclear activities, and the FCO said it was "seriously concerned" by the development.
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies