The UK government summoned the Iranian ambassador on Thursday after news that imprisoned British-Iranian aid worker Nazanin Zaghari-Ratcliffe is to be recalled to court in Iran, the Foreign Office said.
British authorities conveyed their grave concerns to Hamid Baeidinejad and called for Iran to end Ms Zaghari-Ratcliffe's arbitrary detention.
"We have made it clear to the Iranian ambassador that his country's treatment of Nazanin Zaghari-Ratcliffe is unjustified and unacceptable, and is causing an enormous amount of distress," the Foreign Office said.
A project manager with the Thomson Reuters Foundation, the dual national was arrested in April 2016 at Tehran airport as she prepared to fly back to Britain with her daughter after a family visit.
She was sentenced to five years in jail after being convicted of plotting to overthrow Iran's clerical establishment.
Her family and the foundation deny the charge.
Ms Zaghari-Ratcliffe was temporarily released from jail in March in response to concerns about the spread of Covid-19 in Iran's prisons but her movement is restricted and she is barred from leaving the country.
She was told of a new charge in September, Iranian state television reported.
