The global nuclear watchdog's board of governors has formally declared Iran in breach of its non-proliferation obligations for the first time in 20 years.
Nineteen of the 35 countries on the board of the International Atomic Energy Agency (IAEA) voted for the motion, which was submitted by the US, UK, France and Germany.
It says Iran's "many failures" to provide the IAEA with full answers about its undeclared nuclear material and activities "constitutes non-compliance". It also expresses concern about Iran's stockpile of enriched uranium, which can be used to make reactor fuel but also nuclear weapons.
Iran condemned the resolution as "political" and said it would open a new enrichment facility.
It follows a report from the IAEA last week which criticised Iran's "general lack of co-operation" and said it had enough uranium enriched to 60% purity, near weapons grade, to potentially make nine nuclear bombs.
Iran insists its nuclear activities are entirely peaceful and that it would never seek to develop or acquire nuclear weapons.
Under a landmark 2015 deal with six world powers, Iran agreed to limit its nuclear activities and allow continuous and robust monitoring by the IAEA's inspectors in return for relief from crippling economic sanctions.
Iran also committed to help the IAEA resolve outstanding questions about the declarations under its Nuclear Non-Proliferation Treaty (NPT) Safeguards Agreement.
However, US President Donald Trump abandoned the agreement during his first term in 2018, saying it did too little to stop a pathway to a bomb, and reinstated US sanctions.
Since 2019, Iran has increasingly breached restrictions of the existing nuclear deal in retaliation, particularly those relating to production of enriched uranium.
Diplomats said three countries - Russia, China and Burkina Faso - voted against the resolution at the IAEA board's meeting in Vienna on Thursday. Eleven others abstained and two did not vote.
The text, seen by the BBC, says the board "deeply regrets" that Iran has "failed to co-operate fully with the agency, as required by its Safeguards Agreement".
"Iran's many failures to uphold its obligations since 2019 to provide the agency with full and timely co-operation regarding undeclared nuclear material and activities at multiple undeclared locations in Iran... constitutes non-compliance with its obligations," it adds.
As a result, it says, the IAEA is "not able to verify that there has been no diversion of nuclear material required to be safeguarded". The "inability... to provide assurance that Iran's nuclear programme is exclusively peaceful," it adds, "gives rise to questions that are within the competence of the United Nations Security Council".
The Atomic Energy Organisation of Iran (AEOI) and the Iranian foreign ministry issued a joint statement condemning the "political action" by countries who voted in favour of the resolution and insisting that it was "without technical and legal basis".
They announced that Iran would respond by setting up a new uranium enrichment facility at a "secure location" and by replacing first-generation centrifuges used to enrich uranium with sixth-generation ones at the underground Fordo facility.
"Other measures are also being planned," they added.
The resolution could further complicate talks between Tehran and Washington on a new nuclear agreement that Donald Trump hopes will see Iran end its enrichment programme and prevent it developing a nuclear weapon.
A sixth round of talks is due to be held this Sunday in Oman. However, Trump said earlier this week that he was growing less confident of striking a deal.
He also held a reportedly tense phone call with Israeli Prime Minister Benjamin Netanyahu, who has long argued for a military rather than diplomatic approach. Israel considers the prospect of a nuclear-armed Iran as an existential threat.
It comes amid mounting tensions in the Middle East, with the US advising non-essential staff at some of its embassies in the region to leave and reports saying that Israel is ready to launch strikes on Iranian nuclear facilities.
Iran's defence minister has warned that it would respond to any attack by targeting all US military bases "within our reach".