Boris Johnson will meet Iran's President Hassan Rouhani on his second day of talks in the country.
The UK foreign secretary will continue to press for the release of Nazanin Zaghari-Ratcliffe, who has been held in Tehran since April 2016.
On Saturday, Mr Johnson held "frank" talks within his Iranian counterpart for two hours on a range of subjects.
During the meeting with Mohammed Javad Zarif, he urged the release of Mrs Zaghari-Ratcliffe on humanitarian grounds, along with other dual nationals held in Iran.
Mrs Zaghari-Ratcliffe was arrested on a visit to see her parents with her baby daughter Gabriella and accused of spying - which she denies.
After the arrest, her daughter's passport was confiscated and for the last 20 months she has been living with her maternal grandparents in Iran.
The case was further complicated when Mr Johnson erroneously told a parliamentary committee in November that Mrs Zaghari-Ratcliffe had been in Iran to train journalists.
The foreign secretary later apologised in the Commons, retracting "any suggestion she was there in a professional capacity".
Relations between the UK and Iran have long been difficult. Mr Johnson's visit is only the third by a British foreign minister to Iran in the last 14 years, reports BBC..