Theresa May has announced she remains committed to the Iran nuclear deal after Donald Trump said he was pulling the United States out of the accord.
The US president said he would impose the "highest level" of economic sanctions on Iran as he claimed the state was on the brink of acquiring nuclear weapons.
Iran said it would enrich uranium "more than before... in the next weeks" if negotiations failed over the deal.
Israeli Prime Minister Benjamin Netanyahu backed Mr Trump, describing the agreement as a "recipe for disaster".
Britain, France and Germany had made strenuous attempts to persuade the US president to preserve the deal.
But Mr Trump said the agreement was "disastrous" and a "great embarrassment" to him.
Speaking in the White House, Mr Trump said: "The Iran deal is defective at its core. If we do nothing, we know exactly what will happen.
"In just a short period of time, the world's leading state sponsor of terror will be on the cusp of acquiring the world's most dangerous weapon.
"Therefore, I am announcing today that the United States will withdraw from the Iran nuclear deal."
In a joint statement, Mrs May, German Chancellor Angela Merkel and French President Emmanuel Macron said the decision was a matter of "regret and concern" and said they remained committed to the deal.
They said: "It is with regret and concern that we, the leaders of France, Germany and the United Kingdom take note of President Trump's decision to withdraw the United States of America from the Joint Comprehensive Plan of Action.
"Together, we emphasise our continuing commitment to the JCPoA. This agreement remains important for our shared security."
They urged Iran to "show restraint" in response to the decision by the US.
Talking to Julia Hartley-Brewer on the Breakfast Show on Wednesday morning, foreign policy specialist Professor Scott Lewis told talkRADIO that “the implications, is that Trump and some of the advisors around him have chosen confrontation over containment.
"They’re not seeking to revise this deal or renegotiate it, the idea that Trump can get you a better deal, you can kick that into touch.
"Instead, what they’re looking to do is break Iran economically, and we know, since the 1979 revolution there’s always been tension between the US and the Islamic Republic, but there are some advisors in the States who say you have to change the regime over there. Iran’s Achilles heel is its economy, and for someone like Trump’s new security advisor John Bolton, this is the way to take them down."