Tehran has threatened to break the uranium stockpile limit set in the 2015 deal within the next 10 days, in the latest escalation of tensions with the international community.
The move comes against the backdrop of attacks on oil tankers in the Gulf of Oman, blamed by the US and Saudi Arabia on Iran, which has denied responsibility.
Foreign Secretary Jeremy Hunt has warned of a “great risk” of a drift to war as a result of the attacks, but Labour leader Jeremy Corbyn has urged caution, arguing that there is no “credible evidence” at this point of Iran’s involvement.
Tehran’s announcement that it is ready to breach stockpile limits on low-enriched uranium, which can be used in a nuclear reactor but not an atomic bomb, appears to be an attempt to put pressure on European countries to grant the country access to international financial systems which would enable it to work round US sanctions.
Downing Street declined to spell out the detail of the options to be considered if the limit is breached. Security officials were meeting on Monday to discuss the situation.
But the Joint Comprehensive Plan of Action (JCPOA) agreed in 2015 sets out a series of steps which could be implemented in response to a failure by one party to meet its commitment, starting with diplomatic pressure and moving up to sanctions.
Prime Minister Theresa May’s official spokesman said: “We have been clear about our concern at Iranian plans to reduce compliance with the JCPOA.
“Should Iran cease meeting its nuclear commitments, we would then look at all options available to us.”
The spokesman confirmed that the UK believes it is “almost certain” that last Thursday’s tanker attacks were carried out by Iran.
The UK remains in close co-ordination with international partners to try to find a diplomatic solution to the current tensions, he said.
“Unintended escalation would not be in any party’s interests,” said the PM’s spokesman.
Asked if warships could be deployed to protect merchant shipping, he replied: “We have a number of military assets in the region, including our new naval base in Bahrain and our facility in Oman.|”
But he stressed that Royal Navy ships currently active in the area were taking part in a pre-planned training operation which was “not related to the current situation”..
The PM’s spokesman said: “As the Foreign Office announced on Friday, the UK’s assessment has concluded that it is almost certain that the Iranian Revolutionary Guard corps attacked the tankers.
“No other state or non-state actor could plausibly have been responsible.
“These latest attacks build on a picture of destabilizing behaviour and pose a serious danger to the region. That is why we have called on Iran to cease destabilizing activity.”