Tehran's judiciary chief on Friday dismissed hopes of a swift release, possibly in time for Christmas, for British-Iranian Nazanin Zaghari-Ratcliffe, who is serving a five-year jail sentence for alleged sedition.

Gholamhossein Esmaili said there was no possibility the Thomson Reuters Foundation employee could be freed until she had been tried on additional charges filed last month of "spreading propaganda".

"Nazanin Zaghari has two cases -- in the first, she has been convicted, but the second has yet to go to court and there is no verdict," Esmaili told the judiciary's Mizanonline news service.

"The court can convict or acquit her. If she is convicted, we don't know what the sentence will be. So we don't know when she will be able to be released."

Zaghari-Ratcliffe was arrested at Tehran airport on April 3, 2016, after visiting relatives with her young daughter.

Iranian authorities accused her of links to mass protests in 2009, which she denies, and sentenced her to five years in jail for sedition. They do not recognise dual nationality.

Her husband Richard Ratcliffe told AFP on Thursday that he had been told by an Iranian lawyer that she was now "marked for early release".

"He said there's still paperwork to finalise, but it should be over soon -- days to weeks, not months or years."

On her being home in time for Christmas, Ratcliffe said there was "an outside shot".

Her case has become highly politicised, especially after a "slip of the tongue" by British Foreign Secretary Boris Johnson last month when he stated that Zaghari-Ratcliffe had been training journalists in Iran, which has been used by prosecutors to help justify the new charges.

Johnson visited Tehran earlier this month to press for her release on humanitarian grounds.

An online petition in her support has collected almost 1.5 million signatures.