Dr Olli Heinonen, a former deputy director general of the International Atomic Energy Association, believes the rogue state has enough components to make viable atomic weapons.
The regime’s nuclear bid halted under the 2015 Joint Comprehensive Plan of Action.
Britain, France and Germany still cling to the deal but Tehran announced last year that it would no longer be abiding by its terms following the US withdrawal in 2018.
Since then, it has increased the number of centrifuges that had been dismantled under the deal. These are used to enrich uranium and allow it be used in nuclear weapons.
Dr Heinonen told a National Council of Resistance of Iran conference: “Iran has not only increased its number of centrifuges, but produced about one tonne of low enriched uranium, enriched up to 4.5 percent.
“This amount is actually enough to make one nuclear device if Iran wants to enrich it further to the level of 90 percent.
“At the same time, Iran has installed additional centrifuges and is testing new models of centrifuges.”
More elaborate centrifuges will take years to develop but Iran has 2,000 lower capacity centrifuges.
Dr Heinonen added: “If the 2,000 centrifuges are reassembled it will double the Iranian enrichment capacity.
“It will bring the breakout time – the time that it takes to produce highly enriched uranium for a nuclear weapon – to perhaps only two or three months.
“This is certainly a matter of concern to the international community.”
He told the conference he believed Iran had consistently broken the terms of the Joint Plan and non-proliferation treaties, even before President Trump walked away from the deal.
He added: “I had the opportunity to study the atomic archives discovered in 2018.
“They indicated that Iran had not really dismantled its nuclear weapons-related research and development.Why? “Iran has provided no access nor explanation.
Not only is Iran in non-compliance with its own comprehensive safeguard agreement which was concluded under the Non-Proliferation Treaty, but it most likely has undeclared uranium in its possession.”
In October, Iran hoped to see the lifting of a five-year UN arms’ embargo imposed.
The unravelling of the 2015 nuclear deal, however, is likely to mean its extension for a further five years.
Robert Joseph, former US under secretary of state for arms control and international security, said: “The fundamentally flawed nuclear agreement not only failed to close off Iran’s pathways to a nuclear weapon, it paved the way for a future nuclear-armed Iran.
“In the interim, it provided the regime with billions of dollars that it used to expand regional aggression, support terrorist allies, and oppress its own citizens.”
John Rood, former US under secretary of defence for policy, said: “I continue to worry about the potential for conflict emerging with Iran.
“The simmering low-intensity conflict between the United States and Iranianbacked militias came to a boil in January when the US conducted a strike that killed the leader of the Iranian Revolutionary Guard Corps Quds Force, Qasem Soleimani.
“Iran retaliated by launching missiles at a US base in Iraq, injuring 110 US soldiers.
“This exchange was merely the culmination of smaller scale attacks and US retaliation that have led the US and Iran to the brink of a large-scale conflict.”