Iran has accused Britain of breaking international law by taking the rare step of granting diplomatic protection to a dual British-Iranian mother jailed in Tehran over spy allegations.
UK Foreign Secretary Jeremy Hunt said on Friday that London's decision to extend the special status to Nazanin Zaghari-Ratcliffe was aimed at sending a "very strong message" to Iran.
But Iran's envoy to London said the decision "contravenes international law".
"UK Govt's extension of diplomatic protection to Ms Zaghari contravenes int'l law. Govts may only exercise such protection for own nationals," Hamid Baeidinejad tweeted late Thursday.
"As UK Govt is acutely aware, Iran does not recognize dual nationality. Irrespective of UK residency, Ms Zaghari thus remains Iranian," he wrote.
Zaghari-Ratcliffe has suffered from a range of health issues since being arrested while leaving Tehran after taking her infant daughter on a family visit in April 2016.
She was sentenced to five years for allegedly trying to topple the Iranian government. She has denied the charges.
The 40-year-old went on a brief hunger strike in January to protest a lack of proper medical care in jail.
Her husband Richard said on Friday she was still suffering from "neurological problems".
"She was taken for an initial assessment to a doctor, who said she was surprised she was in prison given the health condition she was in," he told BBC radio.
"So she's obviously not in a great position."
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