Foreign Secretary Jeremy Hunt has agreed Britain will grant "diplomatic protection" to Nazanin Zaghari-Ratcliffe, the British-Iranian mother held in Iran where she is accused of spying.
Mr Hunt said he had decided to invoke the rarely used diplomatic device in recognition of Iran's failure to meet its obligations under international law in its treatment of the 41-year-old.
While he acknowledged the move was unlikely to be a "magic wand" that secures her immediate release, he said it elevated the dispute over her detention to a formal state-to-state issue.
The announcement was welcomed by her husband, Richard Ratcliffe, who has been lobbying ministers to take the step for some time.
"It is a very clear statement and so hopefully the Iranian authorities will realise this has gone on too long. They cannot play games like this with ordinary people's lives," he said.
In reaching his decision, Mr Hunt said 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.
'A magic wand'
The Foreign Secretary said Tehran's actions had been "totally wrong" and that no state was entitled to use innocent individuals as "pawns for diplomatic leverage".
"This represents formal recognition by the British Government that her treatment fails to meet Iran's obligations under international law and elevates it to a formal state-to-state issue," he said in a statement.
"My decision is an important diplomatic step which signals to Tehran that its behaviour is totally wrong.
"It is unlikely to be a magic wand that leads to an overnight result. But it demonstrates to the whole world that Nazanin is innocent and the UK will not stand by when one of its citizens is treated so unjustly.
"I know there are many in Iran who understand the unjustness of this situation. 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."
Mrs Zaghari-Ratcliffe, a charity worker with the Thomson Reuters Foundation, was arrested in April 2016 at Tehran airport as she was returning home to London with her then 22-month-old daughter, Gabriella, following a family visit.
She was subsequently sentenced to five years imprisonment, despite strenuously denying the charges against her, while Gabriella remains in the country cared for by family.
In January, Mrs Zaghari-Ratcliffe went on hunger strike for three days saying the Iranian prison authorities had refused her treatment for a series of medical conditions including lumps in her breasts, severe neck pain, and numbness in her arms and legs.