The Prime Minister and Foreign Secretary ramped up pressure on Iran to release a charity worker jailed in Tehran during talks in New York.
Theresa May told Iranian president Hassan Rouhani she had “serious concerns” about the jailing of Nazanin Zaghari-Ratcliffe.
The British-Iranian mother was sentenced to five years in jail after being accused of spying by Tehran’s Islamist regime, a charge she vehemently denies.
In talks on the fringes of the United Nations General Assembly (UNGA), Mrs May called for Mrs Zaghari-Ratcliffe’s release.
It came after Jeremy Hunt demanded rapid action to secure her freedom in his first face-to-face meeting with Iran’s foreign minister.
The Foreign Secretary told Javad Zarif that Mrs Zaghari-Ratcliffe must be returned home quickly.
During talks on the UNGA sidelines, he said: “I again pressed for Nazanin’s swift release – she deserves to be back at home with her family,” Mr Hunt said in a statement.
It is also “absolutely essential” the Iranian government take steps to ensure the release of “a number of detained dual nationals”, he added.
Mrs Zaghari-Ratcliffe, who works for the Thomson Reuters Foundation, was detained at Imam Khomeini airport in April 2016.
Last month she was granted a three-day release from Evin prison but her request for an extension was denied and she was forced to say goodbye to her four-year-old daughter, Gabriella, and return to jail.
Husband Richard Ratcliffe wrote an open letter to Mr Zarif calling the short release a “cruel game”.
Mrs May also used her meeting with President Rouhani to underline the UK’s commitment to the Iran nuclear deal.
Britain and its European allies responded with dismay to US president Donald Trump’s decision to pull out of the 2015 Joint Comprehensive Plan of Action that relieved sanctions on Tehran in return for an end to Iran’s military nuclear ambitions.