Charity worker Nazanin Zaghari-Ratcliffe, who has been detained in Iran for more than two years, plans to go on a three-day hunger strike in protest at her treatment.
Saturday marked 1,000 days since Mrs Zaghari-Ratcliffe was put in detention, accused of trying to overthrow the Iranian Government, charges she has always denied.
Her family, from Fleet in Hampshire, have been campaigning for her release and say she has been refused access to medical help.
Mrs Zaghari-Ratcliffe, who works for the Thomson Reuters Foundation, was detained at Imam Khomeini airport in April 2016.
In August last year, she was granted a three-day release from Evin prison but her request for an extension was denied and she was forced to say goodbye to her four-year-old daughter, Gabriella, and return to jail.