Jeremy Hunt has warned Iran against using innocent dual-national Britons as political prisoners as he prepared to visit the country for the first time.
The Foreign Secretary called on its hardline Islamic regime to release people including charity worker Nazanin Zaghari-Ratcliffe, currently jailed on what are widely regarded as trumped-up spying charges.
Hunt, the first Western foreign minister to visit Iran since the US pulled out of a nuclear deal and imposed sanctions on Tehran, also called for its leaders to cease "destabilizing activity".
The Foreign Secretary is also due to discuss the war in Yemen during his trip, which will include talks with his counterpart, Mohammad Zarif.
Hunt, who is due to arrive in Iran on Monday, said: "More than anything, we must see those innocent British-Iranian dual nationals imprisoned in Iran returned to their families in Britain.
"I have just heard too many heartbreaking stories from families who have been forced to endure a terrible separation.
"So I arrive in Iran with a clear message for the country's leaders: putting innocent people in prison cannot and must not be used as a tool of diplomatic leverage."