The Islamic republic's cooperation with the agency has therefore come into question, they said.
Information from Libya and other countries had led to the discovery of the designs, which could be used to develop machines to produce weapons-grade material, Western diplomats said, on condition of anonymity.
The diplomats said the designs were for a more sophisticated, second-generation machine developed by the British-Dutch-German consortium Urenco.
There was no suggestion that Urenco had supplied the design, they said. The consortium has previously denied providing Iran with any technology.
"This (discovery) raises very serious questions that we will address at the upcoming (International Atomic Energy Agency) board meeting on (8 March)," a Western diplomat said.
Diplomats said the discovery could give Washington new ammunition in its drive to report Iran to the UN Security Council for concealing the total extent of it nuclear programme.
The Council has the power to impose economic and diplomatic sanctions.
"This raises very serious questions that we will address at the upcoming (IAEA) board meeting"
Washington has called on Iran to follow Libya's example by owning up to running an atomic weapons programme and fully cooperating with IAEA inspectors.
Iran insists that its nuclear programme is purely for peaceful purposes.
In December 2003, under intense international pressure, Iran temporarily halted uranium enrichment and signed the additional protocol allowing snap nuclear inspections.
Western diplomats have previously said there are parallels between Libya and Iran's nuclear programme.
Libya admitted in December it had been seeking nuclear, chemical and biological weapons and invited US, British and UN experts to help it destroy its weapons of mass destruction facilities.
Since then, Tripoli has provided the Vienna-based IAEA with a wealth of detail about it nuclear weapons programme, including designs for nuclear warheads.