Hamid Baeidinejad, the country's top envoy in London, said Iran would "be ready to go back to the previous situation" if America pulls out of the 2015 pact, as threatened by US President Donald Trump.
"When the United States is out of the deal, it means that there is no deal left," he told CNN's Christiane Amanpour.
"Because (an) important party of the treaty has abrogated and violated in clear terms the treaty."
Trump is reportedly poised to scrap the agreement, negotiated in 2015 between Tehran and six world powers, ahead of a May 12 deadline for Washington to renew its support for the deal.
The agreement imposes strict restrictions on Iran's nuclear programme in return for the loosening of economic sanctions.
Baeidinejad said Tehran was exploring various responses to such a move from the US - including restarting nuclear activities.
"It could be enriching uranium, it could be redefining our cooperation with the agency and some other activities that are under consideration," he added.
That is something very real, in fact very realistic.
The ambassador denied Iran would restart any nuclear weapons production.
The country has always denied it sought a nuclear weapon, insisting its atomic programme was for civilian purposes.
European leaders, including French President Emmanuel Macron, have been pressing Trump to strengthen rather than abandon the three-year-old deal that took more than a decade to reach.
Macron, who met Trump in Washington last week to try to convince him to not walk away, admitted Wednesday he still did not know what decision the US leader would take.
Trump has pilloried the agreement designed to curb Iran's nuclear programme as "insane" and "ridiculous" and called for fresh measures to counter Iran's ballistic missile programme and support for militant groups across the Middle East.
On Tuesday, Israeli Prime Minister Benjamin Netanyahu said he had new "proof" of an Iranian nuclear weapons plan that could be activated at any time.
Baeidinejad said international observers' final assessment ahead of the 2015 agreement contradicted Israel's claims.
