An exiled dissident who first blew the whistle on Iran's covert nuclear programme today warned that Tehran has secretly built about 4,000 centrifuges capable of producing weapons-grade uranium.

Alireza Jafarzadeh said that the centrifuges, which he said were unknown to the United Nations' nuclear watchdog, are operational and ready to be installed at Iran's nuclear facility at Natanz.

Mr Jafarzadeh runs Strategic Policy Consulting, a Washington-based think-tank focusing on Iran and Iraq. He said that the information came from sources within the administrative regime who had proved accurate in the past.

The International Atomic Energy Agency (IAEA), which monitors nuclear activity all over the world, is holding an emergency meeting at 3pm BST today, following Iran's decision yesterday to restart work at its uranium processing plant in Isfahan. It did not immediately comment on the allegations.

The agency has previously said that it was aware of the existence of 164 centrifuges - high-speed rotational machines used in separation processes - at Natanz, 300 miles south of Tehran.

Under an agreement with the IAEA, Iran had pledged to stop building centrifuges, which are used to generate nuclear power but have the capacity to enrich uranium to levels high enough to fuel atomic bombs.

America says that Tehran is secretly trying to produce nuclear weapons. Washington has, until now, reluctantly backed a diplomatic strategy led by Britain, France and Germany for a compromise deal allowing Iran to process uranium for the purposes of civilian power generation, but barring it from producing enriched uranium for weapons use.

But Mahmood Ahmadinejad, the Islamic republic’s hardline new President, yesterday announced that he was rejecting the deal, in an act of brinkmanship which has been seen as a deliberate step toward confrontation.

Mr Jafarzadeh said: "The 4,000 centrifuge machines have not been declared to the IAEA, and the regime has kept the production of these machines hidden from the inspectors while the negotiations with the European Union have been going on over the past 21 months." The claims could not immediately be independently verified.

Mr Jafarzadeh said the centrifuges were manufactured in Isfahan and Tehran, and that construction of buildings, concrete foundations and other work needed to prepare the Natanz facility for centrifuge installation has continued in recent months.

The IAEA’s 35-nation board of governors has the power to issue a formal warning to Tehran or to report Iran to the UN Security Council, which has the authority to impose economic or political sanctions on the regime. This latter step was however being seen as unlikely.

In 2002, Mr Jafarzadeh - then a member of the National Council of Resistance of Iran, an exiled opposition group - disclosed information about two hidden nuclear sites that helped to uncover almost two decades of covert Iranian atomic activity and sparked present fears that Tehran wants to build a bomb.

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