The UK has said it remains committed to the Iran nuclear deal, insisting that it has left the world “a safer place”.
In a joint statement issued with her European counterparts, Theresa May said she regretted Donald Trump’s decision to exit the 2015 agreement.
The group, which includes France and Germany, said the accord “remains important for our shared security”.
“We urge all sides to remain committed to its implementation and to act in a spirit of responsibility”, they wrote.
Labour, meanwhile, accused President Trump of a “reckless, senseless and immoral act of diplomatic sabotage”.
Iran has indicated it will seek to continue the deal, to which China, Russia and the European Union are also signed up.
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After speaking together on Tuesday evening, Mrs May, German Chancellor Angela Merkel and French President Emmanuel Macron made clear they would not let the agreement – officially known as the Joint Comprehensive Plan of Action (JCPoA) – collapse.
“Our governments remain committed to ensuring the agreement is upheld, and will work with all the remaining parties to the deal to ensure this remains the case including through ensuring the continuing economic benefits to the Iranian people that are linked to the agreement,” they wrote
“After engaging with the US Administration in a thorough manner over the past months, we call on the US to do everything possible to preserve the gains for nuclear non-proliferation brought about by the JCPoA, by allowing for a continued enforcement of its main elements.”
Failure of diplomacy
Analysis by the BBC’s diplomatic correspondent James Landale
Donald Trump’s decision to withdraw the US from the Iran nuclear deal represents a failure of British and European diplomacy. They were unable to persuade the US president to change his mind.
But it also marks the start of the next and perhaps harder task of trying to hold what is left of the agreement together. All three countries have promised to stay in the deal, known by its acronym the JCPoA.
All three countries have promised to work with the remaining signatories to uphold the agreement. The key question will be how far the UK and other European countries are ready and able to go to protect their banks and firms from US sanctions if they do business with Iran.
A failure of British and European diplomacy
Under the terms of the landmark deal, Iran agreed to limit its nuclear activities in return for the easing of sanctions on its economy.
But Mr Trump said it was “defective” and rather than thwarting the Iranian regime’s ambitions to acquire a nuclear bomb, the “rotten and decaying” agreement had left Tehran on the “brink of a nuclear breakout” which would, in turn, fuel an arms race in the Middle East.
Mr Trump said the US would impose the highest level of sanctions on Tehran and any country that tried to help Iran evade economic sanctions would also be penalised.
The US President said he had engaged with the UK and other allies and partners before making the decision.
“As we exit the Iran deal we will be working with our allies to find a real, comprehensive and lasting solution to the Iran nuclear threat,” he said.
In response, Iran said it would restart uranium enrichment if the agreement could not be salvaged.
The shadow foreign secretary Emily Thornberry said Washington’s decision risked plunging the Middle East into “deeper destruction, chaos and conflict”.
“If the nuclear deal is torn up, we not only destroy that platform for future progress, we risk triggering a rapid nuclear arms race in the Middle East, we risk the hardline theocrats seizing all the reins of power in Tehran, and we risk the descent into an unimaginable conflict with Iran,” she said.