Nazanin Zaghari-Ratcliffe, the British-Iranian mother being held in Iran where she is accused of spying, has been granted diplomatic protection by Britain.
Here we answer some key questions about what it means.
What is diplomatic protection?
Granting diplomatic protection is a rarely used diplomatic device which elevates a dispute from being a consular matter to being a formal state-to-state issue.
Under international law, it is a way for a state to take action on behalf of a national whose rights have been breached by another country.
In this case, Foreign Secretary Jeremy Hunt said diplomatic protection was invoked in recognition of Iran's failure to meet its obligations under international law in its treatment of Mrs Zaghari-Ratcliffe.
Diplomatic protection can take the form of consular action, political and economic pressure, negotiations with the other state, judicial proceedings or other forms of peaceful dispute settlement.
It is distinct from diplomatic immunity which covers the status of accredited diplomats.
What does it mean for Mrs Zaghari-Ratcliffe?
Mr Hunt acknowledged the move was unlikely to be a "magic wand" that secures her immediate release.
But he said it demonstrated to the "whole world" that the UK would not stand by when one of its citizens was treated unjustly.
While diplomatic protection does not automatically dictate any particular course of action, the Foreign Office has indicated it will take the steps it believes are most likely to secure the goal of Mrs Zaghari-Ratcliffe's release.
What will happen next?
Mr Hunt said that in reaching his decision, he had taken into account Mrs Zaghari-Ratcliffe's "unacceptable treatment" by Iran since she was detained in 2016, including the denial of medical treatment and the lack of due process in the proceedings against her.
He said Tehran’s actions had been "totally wrong" and that no state was entitled to use innocent individuals as "pawns for diplomatic leverage".
"I know there are many in Iran who understand the unjustness of this situation," he said.
"No government should use innocent individuals as pawns for diplomatic leverage so I call on Iran to release this innocent woman so she can be reunited with her family."