By Bethany Bell
BBC News, Vienna
Diplomats say Iran has given the UN nuclear agency a document containing design information which could be used for parts of a nuclear weapon.
Iran has denied its nuclear work is intended to develop weapons
The IAEA has been asking Iran for the design document since 2005.
The move comes as Mohammed ElBaradei, the head of the agency, the IAEA, is finalising his latest report on Iran.
The report is expected to shed light on what questions Iran has answered about its nuclear work and could determine if new UN sanctions are imposed on Tehran.
The design document contains information on casting uranium metal, a process related to the fabrication of nuclear weapon components.
Iran says it was given the papers without asking for them during its black market purchases of nuclear equipment years ago.
But the document is one of a number of issues to have raised suspicions about Iran's nuclear ambitions.
IAEA inspectors have been permitted to view the document in Iran, but up to now, Teheran has refused to let them have a copy.
Now a document has been handed over to the IAEA.
Some diplomats see this as a sign of progress that Iran is co-operating with the IAEA and is revealing details about its past nuclear activities.
Others are sceptical, seeing this as a last minute attempt by Iran to stave off the threat of more UN sanctions.
One Western diplomat told the BBC that it was not a particular surprise if Iran had handed over the document.
But he said Iran should concentrate on answering all the outstanding questions without delay.
The next report by the IAEA on Iran, which is expected in the next day or two, could help determine whether or not world powers will move ahead with tougher sanctions against Tehran.