'Every alternative is worse': UK Foreign Secretary urges Trump to preserve nuclear deal
The call came in an op-ed piece in The New York Times, ahead of a meeting with officials from the US administration in Washington on Monday.
"At this delicate juncture, it would be a mistake to walk away from the nuclear agreement and remove the restraints that it places on Iran," Johnson wrote in the Times piece.
He argued inspectors from the International Atomic Energy Agency have been granted extra powers to monitor Iran’s nuclear facilities, "increasing the likelihood that they would spot any attempt to build a weapon".
Trump has threatened to withdraw from the agreement when it comes up for renewal on May 12, demanding his country's European allies "fix the terrible flaws" in it or he will re-impose sanctions on Iran that were eased under the historic accord in exchange for Tehran limiting its nuclear programme.
The nuclear deal was struck in 2015 among Iran and Britain, China, France, Germany, Russia and the United States, then led by Barack Obama.
Johnson added: "I believe that keeping the deal's constraints on Iran's nuclear program will also help counter Tehran's aggressive regional behaviour. I am sure of one thing: every available alternative is worse. The wisest course would be to improve the handcuffs rather than break them."
Johnson will not meet Trump during his trip, but is set to appear on TV news show Fox & Friends, known to be a presidential favourite.
The foreign minister will meet US Vice-President Mike Pence, National Security Adviser John Bolton and foreign policy leaders in Congress.
As the deadline looms, Iran's President Hassan Rouhani said on Monday Tehran would be willing not to abandon the nuclear deal even if the United States pulls out, providing the European Union offers guarantees that Iran would keep benefiting from the accord.