The husband of Nazanin Zaghari-Ratcliffe, a British-Iranian woman detained in Tehran for sedition, on Wednesday called on the UK government to do "everything to protect her" after Iran announced she faces fresh charges.
Zaghari-Ratcliffe, 41, has spent more than four years in jail or under house arrest since being arrested in the Iranian capital in April 2016 while on a visit to see family with her young daughter.
Iranian state television's website Iribnews said on Tuesday she and her lawyer had been notified of a new indictment, without giving further details or a trial date.
But her husband, Richard Ratcliffe, said she is due to appear in court on Sunday and it was "increasingly clear" she was being held as a "hostage" and as "leverage against a UK debt.
"It is important that the UK government does everything to protect her and others as Iran's hostage diplomacy continues to escalate," he added in a statement.
"This starts with the British embassy insisting it is able to attend Nazanin's trial on Sunday."
Zaghari-Ratcliffe, who worked for the Thomson Reuters Foundation -- the media organisation's philanthropic arm -- denied sedition but was convicted and jailed for five years.
The head of the foundation, Antonio Zappulla, on Tuesday condemned the latest move by the Iranian authorities to prolong her detention as "inhumane and unjust".
- 'Bargaining chip' -
British Prime Minister Boris Johnson's spokesman said the UK government was raising concerns with Iran "at the highest levels".
The foreign ministry said bringing new charges was "indefensible and unacceptable", while Ratcliffe said it was clear the Iranian authorities were blocking her release.
"They (the charges) are obviously a bad sign because it's a new court case," he told AFP in an interview in London.
"New court cases in the Revolutionary Court always end in a conviction. We don't know how big that conviction will be but another conviction means more prison.
"It means it's less likely that she's going to come home."
He added: "My sense of it is that this is political... to put pressure on the British government. She was picked up to put pressure on the British government.
"We've been a bargaining chip all along and I've been clear about the fact that I think she's a hostage."
Ratcliffe called it a "cruel game" but said London's approach to put pressure on Tehran "isn't working". "We need to sit down, and talk through what are the alternatives," he added.
Zaghari-Ratcliffe has been on temporary release from Evin prison in Tehran and under house arrest since earlier this year because of the coronavirus pandemic.
Links have been drawn in the UK and Iran between her detention and a debt dating back over 40 years to when the shah of Iran paid the UK £400 million for 1,500 Chieftain tanks.
When the shah was ousted in 1979, Britain refused to deliver the tanks to the new Islamic Republic but kept the money.