The United States has urged Britain to ditch its support for a 2015 nuclear deal with Iran and instead join forces to counter the global threat it says Iran poses.
Despite opposition from European allies, in May US President Donald Trump pulled the US out of a deal between world powers and Iran under which international sanctions were lifted in return for curbs on Iran's nuclear programme.
Since then, Britain, France and Germany have sought to keep the deal alive, while Mr Trump has prepared new sanctions, saying a broader and more balanced deal is needed.
Iran has denounced the sanctions as "US unilateralism".
US Ambassador to Britain Woody Johnson criticised Iran for funding "proxy wars and malign activities" instead of investing in its economy.
He said Iran needed to make tangible and sustained changes to behave like a normal country.
"Until then, America is turning up the pressure and we want the UK by our side," Mr Johnson wrote in the Sunday Telegraph newspaper.
"It is time to move on from the flawed 2015 deal. We are asking global Britain to use its considerable diplomatic power and influence and join us as we lead a concerted global effort towards a genuinely comprehensive agreement."
Asked about Mr Johnson's article, the British foreign office pointed to comments from Middle East minister Alistair Burt, who last week ruled out Britain going along with the US.
Mr Burt said the deal was an important part of regional security and that, with the European Union, the government was trying to protect British companies from the US sanctions when dealing with Iran.
Britain remained open to talks with the US on how to address concerns about Iran.
Iranian President Hassan Rouhani said last week that Mr Trump's repudiation of the nuclear deal was illegal and Iran would not yield to Washington's renewed campaign to strangle Iran's vital oil exports.
However protests have broken out in Iran as its currency has collapsed in value and inflation has soared.
The protests have often begun with slogans against the high cost of living and alleged financial corruption, but quickly turned into anti-government rallies.