The UK said it will take the "extremely unusual"
move of granting diplomatic protection to British-Iranian mother Nazanin
Zaghari-Ratcliffe jailed in Iran over spying allegations, AFP reported.
Foreign Secretary Jeremy Hunt said the decision was in response to the "unacceptable treatment" experienced by the dual national including a lack of medical care during her three years in detention.
"I have today decided that the UK will take a step that is extremely unusual and exercise diplomatic protection," he said in a statement Thursday.
"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."
Diplomatic protection is a rarely-used mechanism allowing nations to seek protection on behalf of its citizens on the grounds that they have been wronged by another state, according to Britain's foreign office.
Zaghari-Ratcliffe has suffered from health issues, including undergoing tests for breast cancer and a series of panic attacks, while her emotional state has worsened during her confinement.
"We have not even been able to secure her the medical treatment she urgently needs despite assurances to the contrary," Hunt added.
Hunt said his decision Thursday was "an important diplomatic step" signaling to Tehran that "its behavior is totally wrong".
However he conceded that it was "unlikely to be a magic wand that leads to an overnight result" and repeated calls for her release.
"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," he said.
A project manager with the Thomson Reuters Foundation, the media group's philanthropic arm, Zaghari-Ratcliffe was arrested in April 2016 as she was leaving Iran at the end of a holiday with her infant daughter.
She was sentenced to five years in prison in September 2016 for allegedly trying to topple the Iranian government.
Last August, the mother was unexpectedly released for a three-day furlough and was reunited with members of her family including her daughter Gabriella outside the Iranian capital.
Zaghari-Ratcliffe, the Thomson Reuters Foundation and the British government have consistently denied the charges against her.
Prime Minister Theresa May has also called for her release, directly appealing to Iranian President Hassan Rouhani to free the mother at the UN's general assembly in New York last year.
Her husband Richard who has been lobbying Hunt and the previous foreign secretary Boris Johnson for diplomatic protection for his wife since 2017, welcomed the move.
"It is a very clear statement and so hopefully the Iranian authorities will realize this has gone on too long," the UK's Press Association reported him as saying.
"They cannot play games like this with ordinary people's lives."
Her London MP Tulip Siddiq said Iran has "violated numerous international human rights standards" including denial of a fair trial and consular access and holding her in inhumane conditions without proper access to medical care.