British Foreign Secretary Jeremy Hunt said he told his Iranian counterpart on September 27 that Britain will not allow the case of imprisoned British-Iranian aid worker Nazanin Zaghari-Ratcliffe to rest.
"Iran is detaining innocent people as an instrument of diplomatic leverage and we can't accept that," Hunt told Sky News in New York, where he is attending the United Nations General Assembly.
"I had a very, very frank discussion with the Iranian foreign minister...and I made it very clear that our policy to Iran is not settled and there will be consequences if they continue to think that this is a tool of diplomacy," he said.
Zaghari-Ratcliffe, a project manager with the Thomson Reuters Foundation, a charity, was arrested in April 2016 at Tehran airport as she headed 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, a charge she and her family have denied.
"Iran needs to understand that we will not let it rest," Hunt told Sky News.
Zaghari-Ratcliffe's husband Richard Ratcliffe told Sky News he was "pleased" by Hunt's comments. "It's a really important recognition," he said.
Earlier this week, British Prime Minister Theresa May also lobbied for the release of Zaghari-Ratcliffe during a meeting in New York with Iranian President Hassan Rohani, May's office said.
"From a humanitarian perspective, we should all try to help people in jail." Rohani was quoted as saying afterwards by the Fars news agency: "The Iranian government is doing what it can, trying to facilitate visits and access to information. But the trial and verdict are in the hands of the judiciary. It’s the same in all countries."