A Primark customer said she felt like crying after she found a prison identity card and a worker's cry for help in the pocket of a pair of trousers she bought in the high street fashion chain.
Karen Wisinska (28) from Co Fermanagh bought a pair of camouflage trousers from Primark for around £10 in June 2011 but only discovered the pitiful plea last week.
Primark has been strongly criticised in the past for using Far East sweatshops that employ what would be regarded as slave labour in the West to make clothing.
Hundreds of workers making Primark clothes died in 2013 when a Bangladeshi factory collapsed.
The 'SOS' note claims that Chinese inmates in the Xiang Nan Prison in Hubei Province are working "as hard as oxen in the field" for 15 hours a day to produce clothes for export, and are being given food that "wouldn't even be given to dogs or pigs".
"We call on the international community to condemn the Chinese government for the violation of our human rights!" it adds.
The discovery comes just after two shoppers in Wales found extra labels sewn into Primark garments protesting against poor working conditions. One read 'degrading sweatshop conditions' and the other stated 'forced to work exhausting hours'.
"I was up in Belfast for my birthday and went to Primark the next day," Ms Wisinska said.
"I bought a pair of trousers for about £10, but when I got them home the zip was broken and there is no Primark near me so I just put them in the cupboard.
"Last week, when I was going through my summer clothes, I found the trousers and felt something in the buttoned back pocket. It was an ID card of a male worker in his 30s or 40s with a note wrapped round it.
"When I got the note translated I felt so bad, I felt like crying. Slavery is not something I have ever thought of until I found that. It is so sad to think about it, we have it so handy here. I wouldn't usually shop in Primark but I wouldn't go near it again now."
A Primark spokesman said the company had launched an investigation, but expressed scepticism about the note.
"These crop trousers were last ordered by Primark in early 2009 and were last sold in Northern Ireland in October 2009. We find it very strange that this has come to light so recently, given that the trousers were on sale four years ago.
"We will be contacting the customer to obtain the trousers, so we can investigate how this occurred and whether there are issues which need to be looked into.
"Nine inspections of the supplier have been carried out by Primark's ethical standards team since 2009. To be clear, no prison or other forced labour of any kind was found during these inspections."
Patrick Corrigan of Amnesty International described it as "a horrific tale".
"It's very difficult to know whether its genuine, but the fear has to be that this is just the tip of the iceberg ," Mr Corrigan said.