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”.
.@POTUS Trump to the long-suffering people of #Iran: The people of America stand with you. The future of Iran belongs to its people. They deserve a nation that does justice to their dreams. #JCPOA pic.twitter.com/MMeIpZEG8p— Department of State (@StateDept) May 8, 2018
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.
On Monday, Boris Johnson made a diplomatic dash to Washington in a last ditch push to win over the president.
The Foreign Secretary said Mr Trump would be in line for the Nobel Peace Prize if he could fix the agreement.
The Joint Comprehensive Plan of Action (JCPOA) was signed by the US, China, Russia, Germany, France and Britain with Iran in 2015.
Under its terms, Iran is committed to a peaceful nuclear energy programme.