Boris Johnson will urge Iran to free British-Iranian woman Nazanin Zaghari-Ratcliffe from jail when he visits Tehran.
The foreign secretary is expected to travel to Iran in the next few days.
Ms Zaghari-Ratcliffe has been in jail since April 2016 after being accused of spying. She denies the claim.
Supporters of the 38-year-old say that she recently had a health assessment to see if she was fit enough to remain in prison.