Rebecca Gallagher from Swansea said she was shocked to discover the apparent "cry for help" emblazoned across the label of her new purchase.
The 25-year-old has vowed never to wear her bargain find for fear it could be the result of exploitative labour.
"I was amazed when I checked for the washing instructions and spotted this label," she 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.
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"To be honest I've never really thought much about how the clothes are made. But this really made me think about how we get our cheap fashion," she explained.
"I dread to think that my summer top may be made by some exhausted person toiling away for hours in some sweatshop abroad."
Rebecca claimed she called Primark after being disturbed by what she saw, but was "put on hold for 15 minutes before being cut off."
A spokesman for the cut-price high street giant said there had been "no other incidents of this kind," adding: "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."
The retailer also took to Twitter after the story went viral, writing: "We are currently investigating the origins of an additional label which has been found in one of our dresses."
However, their response isn't good enough for some with one Twitter user posting: "Yeah right @Primark investigating for real. Did u have to wait till a label exposed further what slavery goes on in those factories #primark" and another saying: "@Primark if you can afford to charge £10 for dress of course someone is being exploited. I blame your customers too."