Monique Villa said in a statement that Nazanin Zaghari-Ratcliffe's "health is already at its poorest" and that she hasn't received access to serious medical care after discovering lumps in her breasts.
"This is slow and cruel torture, yet one more injustice inflicted upon her," Villa said. Zaghari-Ratcliffe, who worked for the charity arm of Thomson Reuters, is set to begin a three-day hunger strike on Monday. She will be joined by famed imprisoned Iranian human rights activists Narges Mohammadi.
Zaghari-Ratcliffe was arrested in April 2016 in Iran and is serving a five-year prison sentence for plotting the "soft toppling" of its government while traveling with her toddler daughter. Her sentence has been widely criticized.
Last November, British Foreign Secretary Jeremy Hunt raised the case of Zaghari-Ratcliffe with Iranian officials during his visit to the Islamic Republic. Hunt at the time retweeted pictures of him meeting with her family and playing with her young daughter, saying, "No child should have to go this long without their mother."