The US Congress has rejected Iran's choice for ambassador to the United Nations, outraged by the prospect of a member of a group responsible for the 1979 takeover of the US embassy in Tehran stepping on US soil.
The move forces President Barack Obama to make a decision that could have serious diplomatic repercussions.
In a rare unanimous vote, the House of Representatives backed a bill that would bar entry to an individual found to be engaged in espionage, terrorism or a threat to national security. The vote came four days after similar action in the Senate and sends the bill to the White House.
The Obama administration opposes the selection of Hamid Aboutalebi because of his participation in a Muslim student group that held 52 Americans hostage for 444 days during the takeover. US officials have told Iran Aboutalebi is unacceptable, and the State Department has indicated the issue could be resolved if Tehran withdrew the nomination.
Iran has called the rejection "not acceptable", with Foreign Ministry spokeswoman Marzieh Afkham saying Aboutalebi was one of the country's best diplomats and argued that he previously received a US visa. Aboutalebi has insisted his involvement in Muslim Students Following the Imam's Line was limited to translation and negotiation.
The ban comes against the backdrop of ongoing nuclear talks between Iran and the West.
In practical terms, Obama must decide whether to sign or veto legislation that could upset host country agreements with numerous nations. Hours after the House vote, White House officials declined to say what the president would do. Spokesman Jay Carney said the administration was continuing to tell Iran that its choice was unacceptable.
Republican Senator Ted Cruz took the lead on the measure in the Senate, sponsoring it and securing the support of Democrats and Republicans. He pressed Obama to sign the bill into law.
"We, as a country, can send an unequivocal message to rogue nations like Iran that the United States will not tolerate this kind of provocative and hostile behaviour," Cruz said.
Republican congressman Ed Royce said Iran's choice wasn't surprising because Tehran's "primary motive is showing contempt for the United States".
The bill would impose a blanket prohibition to "deny admission to the United States to any representative to the United Nations who has engaged in espionage activities against the United States, poses a threat to United States national security interests or has engaged in a terrorist activity against the United States".