Rebecca Gallagher, 25, who bought the summer dress for 10 pounds ($17), has vowed not to wear clothing from Primark again.
"I was amazed when I checked for the washing instructions and spotted this label," Gallagher told the South Wales Evening Post. "It was stitched by hand to say 'Forced to work exhausting hours' and sewn in with the other normal labels."
Gallagher said she called the store to ask them about it, but was put on hold and then cut off after 15 minutes. Primark wrote an email to the Huffington Post claiming the store had no knowledge of the labels.
"We find it very strange that this has come to light recently, given that dress was on sale more than a year ago, with no incidents of this kind relating to this dress. We would be grateful if the customer would give us the dress, so we can investigate how the additional label became attached and whether there are issues which need to be looked into."
Since Gallagher's find, Rebecca Jones, who does not know Gallagher, reported finding a similar label that read: "Degrading sweatshop conditions."
Karen Wisínska of Northern Ireland also found a note in a garment she bought at the Primark in Belfast. According to Amnesty International, the note was found wrapped around a prison identity card and had "appeal to the international community to condemn the Chinese government for a violation of the prisoner's human rights" written in Chinese.
Wisínska said she bought the pants in 2011, but didn't find the note until this week when she removed the unworn piece of clothing from her drawer.