- Video report by ITV News correspondent Dan Rivers
The UK has "no intention" of walking away from the Iran nuclear deal following Donald Trump's decision to ignore pleas and pull the United States out of the accord, the Foreign Secretary has said.
Giving a statement in the House of Commons on the President's actions, Mr Johnson told how the UK did its utmost to prevent the withdrawal.
Iran's Supreme Leader Ayatollah Ali Khamenei has criticised both Britain and the US, stating that he does not trust the UK and that Trump "will be food for worms" following his decision to reject the deal.
Earlier on Wednesday, the Prime Minister added that she had been clear with Mr Trump "in a number of conversations" that the "nuclear deal with Iran should stay".
Theresa May added that she continued "to believe that the Iran nuclear deal was an important step forward in helping to keep the world safe", and that the UK would remain committed to it.
On Tuesday, Mrs May joined with the leaders of France and Germany to say that it was a matter of "regret and concern" that the US had withdrawn from the pact, and that the three countries were recommitting to the deal.
Announcing his decision to withdraw from the deal, 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.
Tehran immediately responded by warning it would begin enriching more uranium than ever if negotiations with other countries in the pact failed.
Ali Khamenei further challenged the US President over his decision to pull out of the deal, telling him: "You cannot do a damn thing!"
In a joint statement, Theresa May, German Chancellor Angela Merkel and French President Emmanuel Macron said: "Together, we emphasise our continuing commitment to the Joint Comprehensive Plan of Action (JCPOA).
"This agreement remains important for our shared security....
"We urge all sides to remain committed to its full implementation and to act in a spirit of responsibility."
The statement continued that "the world is a safer place as a result" of the Joint Comprehensive Plan of Action (JCPOA).
The three leaders also called on the US "to avoid taking action which obstructs its [JCPOA] full implementation by all other parties to the deal" and asked Iran "to show restraint in response to the decision by the US", adding that it "must continue to meet its own obligations under the deal".
Middle East Minister Alistair Burt added that President Trump's decision meant that there were now "challenges" to "tackle", but the UK, along with France and Germany would be unable to make him change his mind.
He continued that there were "other ways forward and it is our job to make sure those other ways work, and work in a non-confrontational fashion, no matter how difficult that is in a tricky region".
In a demonstration of public anger, Iranian politicians burnt a paper American flag and a paper representing the JCPOA, while chanting "death to America" in the country's parliament.
The chant "Death to America" long has been used in Iran since its 1979 Islamic Revolution. It also has been common to hear it within parliament.
President Hassan Rouhani said he would send foreign minister Mohammad Javad Zarif to visit each of the remaining countries involved in the deal.
Former president Barack Obama, who was one of the signatories to the pact, said the decision was a “serious mistake” that could leave the US facing “a losing choice between a nuclear-armed Iran or another war in the Middle East”.
Iran's regional rival, Israel hailed President Trump for the “bold” move and described the agreement as a “recipe for disaster” that had brought the prospect of war closer and "dramatically increased" Iran's aggression.
Britain, France and Germany had made strenuous attempts to persuade the US President to preserve the deal.
On Monday, Boris Johnson made a diplomatic dash to Washington in a last-ditch push to win over the president.
Following the news of the US's withdrawal from the pact, the Foreign Secretary tweeted that he "deeply regretted" the US decision, but reiterated that the UK remained "strongly committed" to it.
The visit came soon after Ms Merkel and Mr Macron made direct appeals to Mr Trump in face-to-face talks.
But the diplomatic efforts failed to secure even a delay in the announcement.
Speaking in the White House, Mr Trump said the agreement was “disastrous” and a “great embarrassment” to him.
“The Iran deal is defective at its core,” he said.
“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.”
Mr Trump said the move was a sign that the US “no longer makes empty threats”.
“When I make promises, I keep them,” he said.
The Joint Comprehensive Plan of Action 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.