Drugs mule Michaella McCollum Connolly has been moved to a Peruvian jail cell which she must share with 39 other inmates.
The Dungannon woman is understood to have left the cell she has shared with her co-accused Melissa Reid for the past two months at Virgen de Fatima, prison in the capital of Lima.
The women, both aged 20, are now both in a larger cell that holds 40 inmates. The move has raised concerns among their families about the potential impact on their health. It is believed 40 women must share one cold shower and lavatory.
Michaella and her friend Melissa, from Glasgow, have been in custody in Lima since August when they were arrested trying to leave Peru with a quantity of cocaine in their luggage. They were sharing a cell as they waited to hear whether their guilty pleas will be accepted by a judge.
Melissa's father, William, said he is concerned for the women's well-being now that they are living in a more crowded cell.
"They probably had it good in the other cell, but now it's totally different for them," he said. "It's really worrying. It all adds to the concern. I hope they're safe."
Chloe Constant, a French academic specialising in female jails in Peru, said health was the biggest problem in such communal cells.
"When you're living in such close proximity to other prisoners, disease can spread quickly," she told a Sunday newspaper.
"Skin diseases are very common, especially fungal infections. Tuberculosis is also a big problem, and even HIV.
"There is great poverty inside these jails, and that will be a shock."
She added the pair might be transferred to the tough Ancon 2 prison in the desert outside Lima.
The women were caught in August smuggling £1.5m worth of cocaine out of Peru. Michaella and Melissa initially claimed they were kidnapped, held at gunpoint and forced to board a flight from Lima to Spain with 24lbs of cocaine hidden inside their luggage when they were arrested. The pair had been working in Ibiza when they ended up in Peru on a drug smuggling trail.
Last month they pleaded guilty to drug smuggling when they appeared before a judge.