Foreign Secretary Boris Johnson has stepped up pressure on the US not to tear up the international nuclear deal with Iran, which he said had "undoubtedly made the world a safer place".
Mr Johnson spoke by phone with US Secretary of State Rex Tillerson on Tuesday, days ahead of an expected announcement by President Donald Trump that he will refuse to certify the 2015 deal.
Mr Trump repeatedly denounced the deal - under which Iran agreed to give up nuclear weapons programmes in return for the lifting of economic sanctions - during his election campaign, and he last month denounced it at the United Nations General Assembly as "an embarrassment to the United States". However, other signatories including the UK, France and Germany are firmly in favour of keeping the agreement intact.
Failure to certify the so-called Joint Comprehensive Plan of Action (JCPOA) would trigger a 60-day period during which the US Congress would have to decide whether to re-impose sanctions.
Mr Johnson was meeting Iranian Vice President Dr Ali Akhbar Salehi in London on Wednesday to press for Iran's continued compliance with the agreement.
Speaking ahead of the meeting with Dr Salehi, the Foreign Secretary said: "The nuclear deal was a crucial agreement that neutralised its nuclear threat. The UK supports the deal and stresses the importance of all parties continuing to uphold their commitments.
"We have made no bones about our deep concern at Iran's destabilising regional activity, including its ballistic missile programme, but I remain steadfast in my view that the nuclear deal was an historic achievement that has undoubtedly made the world a safer place.
"It was the culmination of 13 years of painstaking diplomacy and has increased security, both in the region and in the UK. It is these security implications that we continue to encourage the US to consider."
Foreign Office political director Karen Pierce met French, German and EU counterparts on Tuesday evening to discuss the European position on the JCPoA.
Mr Johnson spoke by telephone with Iranian foreign minister Javad Zarif to underline the benefits of the nuclear deal.
The Foreign Secretary also raised concerns about the detention in Iran of all dual UK-Iranian nationals, including Nazanin Zaghari-Ratcliffe who was arrested as she tried to leave Tehran last year with her daughter Gabriella following a holiday.
The charity worker was accused of plotting to topple the regime, which she denies, and later sentenced to five years in prison.