Officials from the UN's nuclear watchdog agency will supervise the removal of the United Nations seals, the first step towards restarting central Iran's Isfahan Nuclear Conversion Facility, Ali Agha Mohammadi, spokesman for Iran's Supreme National Security Council, said in a report from the official IRNA news agency.
Reprocessing uranium is a step below uranium enrichment, which is to remain suspended, said Mohammadi. The United States claims the Iranian nuclear programme is designed to produce weapons, a claim Iran denies.
Iran maintains its suspension of uranium enrichment last November was voluntary, and that it has the right to resume the activities at any time.
The move was one of several made on Monday by Iran to put European Union negotiators on notice that Tehran would restart such activities.
It could also lead to Iran being hauled before the United Nations Security Council to face sanctions, as previously called for by the United States.
Iran's apparent decision to call off its nuclear freeze sparked an immediate warning from the European Union, which said any move to restart enrichment would damage EU-Iran trade talks.
"We expect Iran to live up to the commitment of the Paris agreement" of nuclear talks with the EU, said European Commission spokesman Stefaan De Rynck.
Uranium ore concentrate will be
converted into uranium gas
Work to resume at Isfahan involves converting uranium ore concentrate, known as yellowcake, into uranium gas, the feedstock for enrichment.
Iranian officials made clear that Iran will not resume the more important step of actual enrichment - injecting uranium gas into centrifuges used to enrich uranium - in a separate plant in Natanz, central Iran.
Uranium enriched to high levels can be used for nuclear bombs; at low levels it is used as fuel for electricity-producing nuclear power plants.
Earlier on Monday, Iran's parliamentary speaker said Tehran was giving European negotiators until 5pm local time (1230 GMT) to submit an incentives package to Iran before it would announce any such resumption.
But Iran's apparent decision to restart reprocessing appeared to override the deadline.
Speaker Ali Hadad Adel: Iran
remains keen on Europe dialogue
The move could trigger calls from European and American officials to haul Iran before the UN Security Council for possible sanctions.
European diplomats said on Sunday that if Isfahan were restarted, an emergency IAEA board meeting would be called to set a deadline for the Iranians to "see the error of their ways" and stop their enrichment activities.
If such a deadline were not met, a Security Council referral was a likely next step, the officials said.
Iranian leaders have signalled an intensifying impatience with the slow pace of negotiations with Europe, and an incoming conservative administration in Tehran has showed signs of wanting to harden the country's stance.
Iran was particularly annoyed that Germany, France and Britain called for a delay until 7 August in presenting a new offer to sway Tehran away from its enrichment programme.
Keen on dialogue
The three European countries, which have been leading US-backed EU negotiations, said on Monday that European negotiators planned to submit their proposal for Iran's atomic programme "in a few days."
German Foreign Ministry spokesman Jens Ploetner said the deadline for their proposal, aimed at persuading Iran to permanently freeze parts of its contentious nuclear programme particularly uranium enrichment, had never been more specific than "the end of July, early August."
Earlier on Monday, Iran's parliamentary speaker Gholam Ali Hadad Adel said his country did not want to end dialogue with Europe.
"We are willing to continue dialogue with them after we resume part of our nuclear activities," he said. "Iran will not give in to any further waste of time."
On Sunday, an official from the UN's International Atomic Energy Agency in Vienna said the Europeans would present their proposal to Iran next week.
The proposal, which still being finished, is a "generous" offer, including nuclear fuel, technology, other aid and "security guarantees" that Iran will not be invaded if it permanently halts uranium enrichment and related activities, European and Iranian officials confirmed.