By Jonathan Marcus
BBC diplomatic correspondent
The Arak plant in 2002 (image: DigitalGlobe)
A leaked Iranian memo published by The Times newspaper suggests that Iran was actively working on the components of a nuclear bomb as recently as 2007.
Iran has always insisted its nuclear programme is for peaceful purposes.
The BBC is unable to verify the authenticity of the document itself but, if is genuine, it reveals some uncomfortable issues for the Iranians.
The timing could not have been worse, raising questions about if a diplomatic solution to the standoff is possible.
The Times newspaper of London has obtained what appears to be an internal Iranian technical document, that refers to a neutron source - uranium deuteride or UD3 - which experts say has no place in a peaceful nuclear programme and could only be related to weapons research.
Benefit of doubt
Uranium deuteride could be used as what is called a neutron initiator to trigger the chain reaction that gives a nuclear weapon its devastating power.
Both Pakistan and China have explored the use of uranium deuteride in this role.
Though the BBC is unable to verify the authenticity of the document, experts are in agreement that if it is what the newspaper says it is, then Iran has some explaining to do.
Western governments have always believed that Iran had carried out research related to the design of a nuclear weapon but they differ as to whether this research effort was abandoned and if so when.
If this research is continuing then it raises serious questions as to whether there can be any diplomatic solution to rolling back Iran's nuclear programme.
The timing of this leak is also significant - Iran is seen to have rejected a new plan to increase confidence in its nuclear activities that would have involved outside countries helping it with the enrichment of nuclear material for a reactor making medical isotopes.
Western governments are now trying to press Russia and China to go along with tougher economic sanctions.
If these documents are genuine, then even Moscow and Beijing may be far less willing to give Teheran the benefit of the doubt.