The international nuclear watchdog, the IAEA, says Iran has completed the necessary steps to start implementing the nuclear deal, paving way for the lifting of international sanctions.
Director General Yukiya Amano said it was "an important day for the international community".
The agency's inspectors on the ground had verified that Iran had carried out the measures required, he said.
A deal between Iran and world powers was agreed in July last year.
Lifting sanctions would unfreeze billions of dollars of assets and allow Iranian oil to be sold internationally.
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As part of the deal, Iran had to drastically reduce its number of centrifuges and dismantle a heavy-water reactor near the town of Arak, both of which could be used in creating nuclear weapons.
Iran has always maintained its programme is peaceful, but opponents of the deal - such as some US Republicans - say it does not do enough to ensure the country cannot develop a nuclear bomb.