Britain says it will grant "diplomatic protection" to Nazanin Zaghari-Ratcliffe, a U.K.-Iranian dual citizen imprisoned in Tehran since 2016, citing "unacceptable treatment" during her three years of detention.
"I have today decided that the U.K. will take a step that is extremely unusual and exercise diplomatic protection," Foreign Secretary Jeremy Hunt said in a statement on March 7.
"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.
Hunt added, however, that it was "unlikely to be a magic wand that leads to an overnight result," and he reiterated calls for Tehran to immediately set her free.
"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," Hunt said.
"Diplomatic protection" differs in international law from "diplomatic immunity," which applies to government officials, but it converts the matter to a state-to-state issue from the level of normal consular assistance. No other British citizens have been granted such protection in recent years, according to the Financial Times.
Britain's Foreign Office said that such a diplomatic move, which is rarely utilized, allows countries to seek protection of its citizens on the grounds that they have been wronged by another state.
Hunt said he had "not taken this decision lightly" but considered the "unacceptable treatment" Zaghari-Ratcliffe had experienced during her years in detention.
"We have not even been able to secure her the medical treatment she urgently needs despite assurances to the contrary," he added.
Hamid Baeidinejad, Iran's ambassador in London, wrote on Twitter that the British government's decision "contravenes" international law.
"Govts may only exercise such protection for own nationals. As [the] UK Govt is acutely aware, Iran does not recognize dual nationality. Irrespective of UK residency, Ms Zaghari thus remains Iranian," Baeidinejad wrote.
Zaghari-Ratcliffe, a project manager with the Thomson Reuters Foundation, is serving a five-year prison sentence for allegedly plotting to overthrow Iran's clerical establishment, a charge she and her family have denied.
Richard Ratcliffe in January told The Observer newspaper that his wife was having medical treatment blocked for checking lumps in her breasts, neurological care over her neck pains, and numbness in her arms and legs, as well as for seeing an outside psychiatrist.
In August, Zaghari-Ratcliffe was granted three days of temporary release. But she was returned to prison, and now gets just weekly visits from her daughter.