Prime Minister Theresa May has urged the US to recertify the nuclear deal with Iran because it is “vitally important for regional security”.
In a phone call with President Trump, Mrs May stressed the importance of the deal being “carefully monitored and properly enforced”.
The US President had been expected to scrap the agreement, as he said it did not serve US security interests.
Foreign Secretary Boris Johnson called the deal “an historic achievement”.
He is to meet Iranian Vice President Dr Ali Akhbar Salehi in London on Wednesday.
Speaking ahead of the meeting, Mr Johnson said: “We have made no bones about our deep concern at Iran’s destabilising regional activity… but I remain steadfast in my view that the nuclear deal was an historic achievement that has undoubtedly made the world a safer place.”
In 2015, Iran agreed to reduce much of its nuclear programme and in return economic sanctions on the region were lifted.
Last month, Mr Trump denounced the deal at the United Nations General Assembly as “an embarrassment to the United States”.
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But the prime minister and the president have discussed the need for the UK, US and others to work together to ensure the deal was being upheld on all sides.
Mr Trump and Mrs May agreed to remain in contact ahead of any decision.
During their conversation, Mrs May and Mr Trump also discussed the impact of the Bombardier trade dispute on Northern Ireland.
Earlier this week, Mrs May reaffirmed the UK’s strong commitment to the agreement in a phone call with Israel.