Theresa May will make a personal plea for the released of a jailed charity worker in Tehran during talks with Iran's president.
The Prime Minister has raised "serious concerns" about the detention of Nazanin Zaghari-Ratcliffe and will urge Hassan Rouhani to free her on humanitarian grounds.
The British-Iranian mother, whose family is from Hampshire, has been imprisoned for more than two years for spying - a charge she vehemently denies.
Mrs May will meet with Mr Rouhani for talks in New York at the United Nations General Assembly.
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 Iran's foreign minister Javad Zarif calling the short release a "cruel game".
Jeremy Hunt pledged to do everything possible to secure Mrs Zaghari-Ratcliffe's release after he became Foreign Secretary.
It followed criticism of predecessor Boris Johnson's handling of attempts to free the mother-of-one.
Mrs Zaghari-Ratcliffe's constituency MP Tulip Siddiq welcomed Theresa May's expected intervention.