The release of Nazanin Zaghari-Ratcliffe, a UK-Iraninan dual national held in Tehran, could be in doubt, her husband told the BBC on Saturday, a day before the end of her five-year jail term.

Zaghari-Ratcliffe, who was detained in Iran in 2016, is due to be released on Sunday, the official end of a sentence over charges she plotted to overthrow the regime in Tehran.

Nazanin, now 42, has strenuously denied the accusations with her case becoming a matter of major diplomatic disagreement between Britain and Iran during the five years she has spent separated from her husband and young daughter.

Despite the official end of the sentence, Nazanin’s husband Richard Ratcliffe told the BBC her detention has "the potential to drag on and on”.

"It’s perfectly possible that Nazanin gets a new court case thrown at her,” he said.

In September, Zaghari-Ratcliffe was summoned before an Iranian court and notified of a new indictment against her, Iranian state media reported at the time.

Richard Ratcliffe, who has campaigned for his wife’s return, said the family had "never seen a copy of the charges on which she was sentenced”.

"There is no written documentation on anything,” he added, accusing Iran of preserving "the space to make it up as they go along at every stage”.

"Now the end of her actual sentence — which was once the worst-case scenario — looks like a good outcome, at this point,” he added.

Nazanin has been temporarily released from Tehran’s notorious Evin prison and has under house arrest since the spring due to the coronavirus outbreak.

For four years, however, at Evin she spent time in solitary confinement in windowless cells, declared hunger strikes and had medical treatment withheld.

Media in both the UK and Iran and Richard Ratcliffe have drawn a possible link between Nazanin’s detention and a British debt dating back more than 40 years.

The British government has previously admitted it owes Iran up to £300 million ($390 million), but both countries have denied any link with the Zaghari-Ratcliffe case.