Citing documents from an unnamed intelligence agency, Der Spiegel said Iran had set up a laboratory in a secret tunnel near a nuclear facility in Isfahan. This would be able to produce large amounts of uranium hexafluoride gas which could, in turn, be used to enrich uranium — a vital component for a nuclear bomb.

Orders to build the tunnel were given last month by Ayatollah Ali Khamenei, Iran’s supreme leader, the magazine said.

The claims emerged as Britain, France and Germany warned Iran last night it could face sanctions if it does not agree to freeze key parts of its nuclear programme by tomorrow.

The three have hitherto failed to back calls by America to refer Iran to the United Nations Security Council. Their patience appears to be running out, however, after Tehran last week tried to backtrack over a deal agreed in principle earlier this month.

Under the deal, brokered by Britain, France and Germany, Iran is obliged to accept a complete freeze on nuclear technology that could be used to make weapons-grade uranium. The United States has accused it of trying to develop a bomb.

Iran challenged the terms of the agreement during talks at the International Atomic Energy Agency (IAEA) last week. It wants to be allowed to operate 20 centrifuges, which are used to enrich uranium, for research purposes.

EU officials rejected this, fearing it could boost Iran’s capabilities in a crucial area of nuclear weapons development.

Western countries had expected Iran to back down but despite attempts at mediation by Jack Straw, the foreign secretary, the talks were adjourned on Friday without agreement.

Kamal Kharrazi, the Iranian foreign minister, said yesterday that Iran was sticking to its demand for an exemption. “The centrifuges will work under IAEA supervision and will be for research purposes only,” he said. The IAEA’s board meets again tomorrow.

The administration of President George W Bush is wary of European attempts to broker a deal. In his most positive comments to date on the initiative, Bush praised Britain, France and Germany for their efforts — but said that any agreement would need to be monitored to ensure Iran was honouring the terms.

Iran’s latest wriggling has compounded concerns that Tehran, which has repeatedly failed to come clean about its activities during a two-year IAEA investigation, is trying to find a way of continuing clandestinely with its nuclear programme.

Last week the National Council of Resistance of Iran, an opposition group, released details of another site in Laviza, a suburb in northeast Tehran, where it claims that laser enrichment of uranium is under way.

Additional reporting: Tom Walker

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