Theresa May has directly pressured Donald Trump not to tear up the international nuclear deal with Iran, as it is "vitally important for regional security".
Downing Street said the Prime Minister had spoken to the United States' president, days ahead of his expected announcement 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.
A Number 10 spokesman said: "Prime Minister Theresa May tonight spoke with Donald Trump ahead of the US President's upcoming decision on recertifying the Iran nuclear deal (JCPOA).
"The PM reaffirmed the UK's strong commitment to the deal alongside our European partners, saying it was vitally important for regional security.
"The PM stressed that it was important that the deal was carefully monitored and properly enforced.
"Mrs May and the president also discussed the need for the UK, US and others to work together to counter destabilising Iranian activity in the region.
"The PM and president agreed that their teams should remain in contact ahead of the decision on recertification."
Last month Mr Trump called the deal "an embarrassment to the United States" while attending the United Nations General Assembly.
However, other signatories including the UK, France and Germany are firmly in favour of keeping the agreement intact.
Meanwhile, Mrs May again lobbied Mr Trump over US tariffs on plane manufacturer Bombardier, which employs more than 4,000 workers in Belfast, stressing "the importance of the jobs provided by the Bombardier factory to the people and economy of Northern Ireland".
The firm has been hit by a proposed 80% levy on exports following complaints by Boeing that the Canadian-owned company had dumped its C Series jets at "absurdly low" prices.
The Downing Street spokesman added: "The PM also reiterated her condolences to the president in the wake of the terrible shooting in Las Vegas."