Prime Minister 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.
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'm announcing today that the United States will withdraw from the Iran nuclear deal."
Mr Trump said if Iran "continues its nuclear aspirations it will have bigger problems than it has ever had before".
He added: "I want to deliver a message to the long-suffering people of Iran: The people of America stand with you."
Mr Trump said it had been nearly 40 years since the regime "seized power and took a proud nation hostage".
"The future of Iran belongs to its people," he added.
He said Tehran would want to negotiate a new deal.
"When they do, I'm ready, willing and able," he said.
"Great things can happen for Iran."
In a joint statement, Mrs May, German Chancellor Angela Merkel and France 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 (JCPoA).
"Together, we emphasise our continuing commitment to the JCPoA. This agreement remains important for our shared security. We recall that the JCPoA was unanimously endorsed by the UN Security Council in resolution 2231. This resolution remains the binding international legal framework for the resolution of the dispute about the Iranian nuclear programme.
"We urge all sides to remain committed to its full implementation and to act in a spirit of responsibility."
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 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.
Former US President Barack Obama said Mr Trump's decision was "so misguided".
In a lengthy Facebook post, he wrote: "There are few issues more important to the security of the US than the potential spread of nuclear weapons, or the potential for even more destructive war in the Middle East.
"That's why the US negotiated the joint comprehensive plan of action in the first place."