Boris Johnson is travelling to the United States as part of a last-ditch diplomatic effort to persuade Donald Trump not to scrap the Iran nuclear deal.

The US President has fiercely criticised the agreement, which eased crippling economic sanctions on Tehran in exchange for commitments to abandon its nuclear weapons programme.

Iran's president has warned Trump that pulling America out of the nuclear deal with world powers would be a "historic regret."

The Joint Comprehensive Plan of Action (JCPoA) is a pact brokered by Britain, the US, China, Russia, Germany and France, and signed with Iran.

In October, President Trump sparked alarm after announcing he would not re-certify the deal to Congress because the regime is not living up to the "spirit" of it and has committed "multiple violations" of the agreement.

Previously Trump has criticised the deal as "one of the worst and one of the most one-sided transactions the United States has ever entered into", adding that it gave Iran "an immediate financial boost and over $100 billion [£74 billion] its government could use to fund terrorism".

In January he issued an ultimatum to "either fix the deal's disastrous flaws, or the United States will withdraw".

If on May 12, the 71-year-old decides to reimpose sanctions on the Middle Eastern country, he would effectively torpedo the international alliance behind the deal.

The Foreign Secretary will travel to Washington on Sunday for two days of talks with senior administration officials including Vice President Mike Pence, as well as national security adviser John Bolton and key foreign policy leaders in Congress.

Earlier this month Johnson stressed the importance of keeping the deal "while building on it in order to take account of the legitimate concerns of the US".

Iranian president Hassan Rouhani on Sunday: "If (the U.S.) opts to pull out of the nuclear deal, it will soon realize that this decision will become a historic regret for them."

Rouhani also assured Iranians that "no change will occur in our lives next week" regardless of Trump's decision.

Iran has faced economic trouble in recent weeks, with some analysts blaming the uncertainty surrounding the accord.

As well as Iran, Johnson's talks in the US are expected to cover North Korea - ahead of President Trump's planned meeting with Kim Jong Un - and the situation in Syria.

The European Union has said the deal "is working and it needs to be preserved".

Ahead of his trip to Washington, Johnson said: "On so many of the world's foreign policy challenges the UK and US are in lockstep.

"We've seen this recently with the response to the poisonings in Salisbury, our strong response to Assad's use of chemical weapons in Syria, and the effort to denuclearise North Korea.

"The UK, US and European partners are also united in our effort to tackle the kind of Iranian behaviour that makes the Middle East region less secure - its cyber activities, its support for groups like Hezbollah, and its dangerous missile programme, which is arming Houthi militias in Yemen."