Supporters of jailed Briton Nazanin Zaghari-Ratcliffe have called on the United Nations to intervene over her alleged torture.
Campaigners say Mrs Zaghari-Ratcliffe, 39, who is entering her 683rd day in captivity in Iran, has been subjected to psychological torture from "the whole armoury of the state".
The dual UK-Iranian national is also claimed to have spent eight-and-a-half months in solitary confinement while she has been repeatedly threatened with having her daughter taken away.
Supporters say this amounts to "psychological abuse", which meets the UN's criteria for torture.
The charity Redress, which has written to the UN special rapporteur on torture on behalf of the Free Nazanin campaign, said evidence "strongly suggests" she is being mistreated.
Redress said her treatment was being inflicted to coerce the British government into securing a deal for her release and to force her to confess or provide information about others.
The letter reads: "While it has serious effects on her physical health, the combined effect of this campaign has been to exert consistently maximum psychological pressure on Mrs Zaghari-Ratcliffe, to demoralise her and put her in a situation of complete powerlessness.
"The treatment has been inflicted on her over an extended period of time, and while she was, and continues to be, at her most vulnerable, a recent mother, in prison abroad and away from her family in the United Kingdom, without consular access and without allowing her family to visit."
Mrs Zaghari-Ratcliffe was detained in April 2016 while visiting her parents with her daughter Gabriella over allegations she plotted to overthrow the government - something she denies.
Foreign Secretary Boris Johnson visited Iran at the end of last year where he held talks with president Hassan Rouhani over the case.
Following this meeting it was reported that Mrs Zaghari-Ratcliffe had become "eligible for early release".