Boris Johnson has urged President Trump not to give up on the Iran nuclear deal, saying “at this delicate juncture it would be a mistake to walk away”.
The foreign secretary is in Washington to persuade the US president to remain a part of the international accord.
The deal saw Iran agree to limit its nuclear activities in return for the easing of sanctions on its economy.
The UK and its European allies have until 12 May to persuade Mr Trump to stick with the deal.
Mr Trump has strongly criticised the agreement, which he calls “insane”.
Britain, France and Germany have been working behind the scenes for weeks in an effort to preserve the deal, which was orchestrated under the Obama administration, and includes Russia and China as signatories.
‘Handcuffs are in place’
Writing in the New York Times, Mr Johnson argued “only Iran would gain” from abandoning nuclear restrictions.
“Of all the options we have for ensuring that Iran never gets a nuclear weapon, this pact offers the fewest disadvantages,” he wrote.
“It has weaknesses, certainly, but I am convinced they can be remedied.
“Indeed at this moment Britain is working alongside the Trump administration and our French and German allies to ensure that they are.”
Mr Johnson said the deal had put restrictions on Iran’s nuclear programme and “now that these handcuffs are in place, I see no possible advantage in casting them aside”.
He added: “The wisest course would be to improve the handcuffs rather than break them.”
While in Washington, Mr Johnson will meet US Vice-President Mike Pence, National Security Adviser John Bolton and foreign policy leaders in Congress.
He will not meet President Trump, but is expected to appear on the Fox & Friends morning news, which Mr Trump is known to watch avidly.
Both German Chancellor Angela Merkel and French President Emmanuel Macron have lobbied Mr Trump directly in recent weeks.
But Mr Trump has threatened to withdraw unless the signatories agree to “fix the deal’s disastrous flaws”. The president believes the terms of the agreement are too lenient.
In a call with Theresa May on Saturday, Mr Trump “underscored his commitment to ensure that Iran never acquires a nuclear weapon”.
Iran’s President Hassan Rouhani says the US will face “historic regret” if it pulls out.
In remarks carried live on state television, he said Iran had “a plan to counter any decision Trump may take and we will confront it”.
The UK-US talks come after Israel revealed “secret nuclear files” accusing Iran of having run a secret nuclear weapons programme, which was reportedly mothballed 15 years ago.
US Secretary of State Mike Pompeo said the documents were authentic and show the 2015 Iran nuclear deal was “built on lies”.
Iran, in turn, accused Mr Netanyahu of lying. Foreign Minister Javad Zarif said the documents produced by Israel were a rehash of old allegations already dealt with by the International Atomic Energy Agency (IAEA), the UN’s nuclear watchdog.
The UK’s ambassador to the US Sir Kim Darroch said the Iran agreement was “a good deal” but efforts were ongoing to “find some language, produce some action that meets the president’s concerns”.
“We think we’re making progress. We haven’t got there yet. We have a few days left to see if we can find a way through,” he said.
He added that Britain and its European partners were also looking at how a deal would work even without the US.