London ? A showdown over Iran’s nuclear weapons program loomed Thursday after British Prime Minister Tony Blair warned that he was prepared to refer the Islamic republic to the U.N. Security Council for possible sanctions if it proceeds with plans to resume uranium reprocessing.

The Bush administration has long favored such sanctions.

“We certainly will support referral to the U.N. Security Council if Iran breaches its undertakings and obligations,” Blair said at his first news conference since his re-election to a third term last week.

Blair’s announcement follows a sharply worded letter to the Iranians from the foreign ministers of Britain, France and Germany warning that any attempt to restart its nuclear program “would bring the negotiating process to an end.”

French Foreign Minister Michel Barnier backed that up with a stern warning of his own. “We continue to hope that Iran will not take this step, the consequences of which it is well aware,” Barnier said Thursday.

Diplomatic efforts to wean Iran from its nuclear program appeared to be nearing the threshold of crisis this week.

After Tehran dispatched a senior negotiator to Vienna, supposedly to deliver a letter notifying the International Atomic Energy Agency of its intention to restart uranium conversion at its plant in Isfahan, the IAEA sent a team of inspectors to Isfahan and prepared to call an emergency session.

Britain, France and Germany quickly ratcheted up the diplomatic pressure with their letter, and now it seems Tehran may have blinked: By late Thursday the Iranian letter had not been delivered to the IAEA.

But Tehran kept up the defiant rhetoric.

“Iran will definitely resume a part of its enrichment activities in the near future … but we are still discussing its conditions and time of restarting the activities,” said Hassan Rohani, Iran’s chief nuclear negotiator.

Iran insists its nuclear ambitions are peaceful.