Britain on Friday warned Iran against throwing detained woman Nazanin Zaghari-Ratcliffe back in jail, after hauling in Tehran's envoy for a dressing-down over her emotive case.
The Foreign Office summoned Ambassador Hamid Baeidinejad on Thursday to hear renewed demands from a senior official for an end to the British-Iranian captive's "arbitrary detention".
Foreign Secretary Dominic Raab told BBC radio Zaghari-Ratcliffe was in a "horrific position", after her husband said Iran had ordered her to report to court for a new trial on Monday and then back to jail.
Britain has made clear to Iran "that is entirely unjustified and totally unacceptable and must not happen", Raab said.
Zaghari-Ratcliffe, who will turn 42 on Boxing Day, has been on temporary release from Tehran's Evin prison and under house arrest since earlier this year because of the coronavirus pandemic.
She has spent more than four years in jail or under house arrest since being detained in the Iranian capital in April 2016 while visiting relatives with her young daughter.
Zaghari-Ratcliffe, who worked for the Thomson Reuters Foundation -- the media organisation's philanthropic arm -- denied charges of sedition but was convicted and jailed for five years.
Her husband Richard Ratcliffe said this week that the Foreign Office's handling of the case "seems disastrous", and that "the UK is dancing to Iran's tune".
Raab told the BBC: "We've made it very clear we want to try to put the relationship between the UK and Iran on a better footing.
"If Nazanin is returned to prison, that will of course put our discussions and the basis of those discussions in a totally different place. It is entirely unacceptable."
Richard Ratcliffe linked the latest development to the postponement of a hearing that was due to take place on Tuesday in London to address Iran's longstanding demand for the repayment by Britain of hundreds of millions from an old military equipment order.
"As Nazanin's husband, I do think that if she's not home for Christmas, there's every chance this could run for years," he said, accusing Iran of "hostage diplomacy".