Boris Johnson is to raise the case of a British-Iranian mother jailed in Tehran with his Iranian counterpart at a summit in Brussels.
The international summit itself will focus on Iran’s nuclear deal.
But the foreign secretary will discuss Nazanin Zaghari-Ratcliffe’s case with Mohammad Javad Zarif when the pair meet for the second time in two months.
Mrs Zaghari-Ratcliffe, from London, has been held in a Tehran prison for 21 months on spying charges she denies.
Mr Johnson will attend alongside French foreign minister Jean-Yves Le Drian, German counterpart Sigmar Gabriel and EU high representative for foreign affairs Federica Mogherini.
He said he would use the opportunity to speak to Tehran foreign minister Mr Zarif about “all of our Iranian dual-national consular cases”.
Last month, Mr Johnson travelled to Iran to press for the release of Mrs Zaghari-Ratcliffe, who was arrested in April 2016 accused of plotting to overthrow the Tehran government.
Charity worker Mrs Zaghari-Ratcliffe, 39, maintains she was on holiday there to introduce her baby daughter Gabriella to her parents.
Mr Johnson was criticised for comments he made to a parliamentary committee in November that Mrs Zaghari-Ratcliffe had been training journalists in Iran – for which he later apologised.
Up to 30 dual nationals from Britain and other western nations are understood to be detained in Iran, including British-Iranian Kamal Foroughi, who was arrested in 2011.
- 3 April 2016: Mrs Zaghari-Ratcliffe is detained at an Iranian airport while travelling home to London with her young daughter.
- April-June 2016: According to Mr Ratcliffe, his wife was subject to “intense interrogation” for the first two months of her imprisonment and kept in solitary confinement
- September 2016: Mrs Zaghari-Ratcliffe is sentenced to five years in prison in Tehran’s Revolutionary Court
- January 2017: She loses an initial appeal against her sentence
- April 2017: Mrs Zaghari-Ratcliffe loses a final appeal in Iran’s supreme court to overturn her sentence
- December 2017: An Iranian database lists her as eligible for release
Mrs Zaghari-Ratcliffe’s family had hoped that she could be released from her five-year sentence after an Iranian database listed her as eligible for release.
Husband Richard Ratcliffe told the BBC in December: “We’re trying not to get too up or too down and just keep battling on”.
The Brussels summit is centred on discussing the Iran nuclear deal, which since 2015 has seen Iran restrict its nuclear programme in exchange for sanctions relief.
Mr Johnson said the deal brought “security and prosperity” to Iran and the world.
“The UK has always been clear: the Iran nuclear deal is a crucial agreement that makes the world safer,” he said.
Mr Johnson said he would also raise recent protests in Iran when he approaches Mr Zarif.
He said he would “make it clear” to the foreign minister that the “right to peaceful demonstration within the law is central to any truly thriving society”.