Primark are investigating an incident in which a shopper discovered a label that read "Forced to work exhausting hours" stitched inside a £10 ($16) dress she had bought from one of the retailer's outlets.
Rebecca Gallagher, 25, has vowed never to wear the dress she purchased in Swansea over fears it was the result of exploitative working conditions.
"You hear all sorts of stories about people working in sweatshops abroad — it made me feel so guilty that I can never wear that dress again," she told the South Wales Evening Post.
Gallagher said she rang Primark to complain but was "put on hold for 15 minutes before being cut off".
She added: "I dread to think that my summer top may be made by some exhausted person toiling away for hours in some sweatshop abroad.
"I've got no idea who put it there but it really took the wind out of my sails. It makes me think that it was a cry for help — to let us people in Britain know what is going on."
Primark said they are taking the allegations "very seriously" but added they find it "very strange" that the complaint has only just arisen as the dress in question was purchased last year.
A spokesperson added: "We are currently investigating the origins of an additional label which has been found inside one of our dresses implying that the garment was produced under poor working conditions.
"Primark takes these allegations very seriously. No other incidents of this kind relating to this dress have been reported.
"We have asked the customer who discovered the label to return the dress, which has not been available for sale since 2013, to us so we can fully investigate how the label became attached and whether there are issues which need to be looked into."