VIENNA, Feb. 2 Iran formally informed the International Atomic Energy Agency today that it will stop all "voluntary" nuclear cooperation with the agency if, as expected, the 35-country agency board reports Iran"s nuclear case to the United Nations Security Council.

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Text: Message of Iran"s Chief Nuclear Negotiator to the Director General of the IAEA (February 2, 2006. PDF document.) The threat, contained in a letter from Ali Larijani, Iran"s chief nuclear negotiator, to Mohamed ElBaradei, the director of the nuclear agency here, means that the agency would no longer be allowed to do voluntary spot inspections and would lose access to key sites and installations.

In addition, Iran has said it will resume its program to eventually build 50,000 centrifuges at Natanz and begin full-scale production of enriched uranium, which can be used to produce electricity or to help build nuclear bombs.

Iran "would have no other choice but to suspend all the voluntary measures and extra cooperation with the agency," the letter, made available to The New York Times, said. "In that case, the agency"s monitoring would extensively be limited and all the peaceful nuclear activities being under voluntary suspension would be resumed without any restriction."

The letter was delivered as the decision-making board opened emergency, closed-door talks today to decide whether to pass a resolution sponsored by Britain, France and Germany that for the first time would open the door to eventual Security Council action against Iran.

The resolution was criticized by the 16-country non-aligned bloc that proposed amendments deleting all references to the Security Council and demanding that Iran"s case remain the responsibility of the United Nations" nuclear watchdog agency here. But the resolution enjoys the support of the United States, Russia and China and is expected to win the majority of votes needed for passage.

In various public statements, Iranian officials, from President Mahmoud Ahmadinejad on down, have threatened to end voluntary cooperation if the decision-making board approves reporting Iran"s case to the Security Council.

But this is the first time that Tehran has delivered its threat in writing to the agency.

The letter did not state when Iran would stop its voluntary activities with the international agency. But an Iranian official said that it would happen "within a day or two" of passage of the resolution, which could come as early as Friday.

This morning Mr. ElBaradei said that the case was not about an "imminent threat."

"We are reaching a critical phase but it is not a crisis situation," he said.

"Whether the board outcome will be to report to the Security Council or not, everybody agrees that the only way to move forward is to go through diplomacy, through negotiation, and there are still windows of opportunity for all concerned parties," he said.

In its current form, the resolution recalls Iran"s "many failures and breaches of its obligations" under the Nuclear Nonproliferation Treaty and "the absence of confidence that Iran"s nuclear program is exclusively for peaceful purposes resulting from the history of concealment of Iran"s nuclear activities."

It asks Mr. ElBaradei to report to the Security Council "on the steps required from Iran" and asks the board to submit all agency reports and resolutions about the nature of Iran"s nuclear activities to the Council.

But in an important concession to Russia and China, which initially resisted any Security Council involvement, the resolution delayed for another month any action in the Council concerning Iran.

The Russians also succeeded in making sure that the resolution did not include the word noncompliance, which they argued had important legal consequences that would automatically require Iran"s case to be referred to the Security Council under the agency"s statutes. But on Wednesday, a senior State Department official maintained that the question was academic. With or without the word, the proposed measure would require the nuclear agency to report to the Security Council all relevant resolutions and findings previously approved, which would include a resolution passed last fall holding Iran in noncompliance.

In Washington on Wednesday evening, one of the chief architects of the Bush administration"s Iran strategy, Robert G. Joseph, the under secretary of state for arms control and international security, detailed the administration"s case against Iran, charging that it was seeking to combine its nuclear ambitions with a fleet of missiles that "can hold hostage cities of our friends in the Middle East and Europe."

Mr. Joseph used a speech to the American-Israel Public Affairs Committee to focus attention on documents discovered by the nuclear agency"s inspectors in Iran that show how to machine-cut enriched uranium into hemispheres, a shape suitable for detonation. Dismissing Iran"s claim that the documents were not part of its nuclear program, Mr. Joseph said, "We know of no application for such hemispheres other than nuclear weapons."

He also described "why we cannot accept a nuclear-armed Iran," in terms that had echoes of the speeches administration officials gave three years ago when they were building a case against Saddam Hussein"s Iraq. "A nuclear-armed Iran could embolden the leadership in Tehran to advance its aggressive ambitions in and outside the region, both directly and through the terrorists it supports," he said.

He said the country, once armed, "would represent a direct threat to U.S. forces and allies in the region," and "could provide the fuse for further proliferation." He also said that it "would represent an existential threat to the state of Israel."

"Finally, Iran is at the nexus of weapons of mass destruction and terrorism," he argued. "If Iran has fissile material or nuclear weapons, the likelihood of their transfer to a third party would increase."

Bush administration officials have said repeatedly that they want a go-slow approach, avoiding sanctions that might enrage the Iranian people, like banning Iran from playing in the World Cup soccer championships, for example.

Reporting for this article was contributed by Sarah Lyall from London, Michael Slackman from Tehran, David E. Sanger and Steven R. Weisman from Washington, and Christine Hauser from New York.