The note, a yellow piece of paper wrapped in a prison identity card, claimed inmates were forced to work 15 hours a day making clothes for export.
The note, translated into English, reads: "SOS! SOS! SOS!
We are prisoners at Xiangnan jail in central China's Hubei province," the BBC reported.
"For a long time, we have been producing clothing for export. We work for 15 hours each day. What we eat is even worse than food for pigs and dogs. The work we do is similar to (the hard work) that oxen and horses do.
"We urge the international community to denounce China for this inhumane act," the note said according to the report.
Karen Wisnska, who lives in Northern Ireland, said she bought the trousers in Primark's Belfast store in June 2011 but had never worn them.
She had put them into her wardrobe and did not take them out again until last week, when she was packing clothes for a holiday and discovered the note.
She was in "in shock" and "felt sick" when she received the rough English translation of the note and contacted Amnesty International, which has documented the use of forced labour in Chinese detention facilities in the past.
"It is a horrific tale," Amnesty's Northern Ireland programme director, Patrick Corrigan said.
"It's very difficult to know whether it's genuine, but the fear has to be that this is just the tip of the iceberg."
A Primark spokesperson said the firm has begun a probe.
Primark is among large clothing firms that have come under close scrutiny in recent years over how and where they source their products.
In a statement to the BBC, the clothing retailer said: "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."
It's not the first time western consumers have found distressing notes allegedly from abused workers in detention in China.
In 2011, a woman in the United States found a letter in a mix of broken English and Chinese inside a Halloween decoration purportedly from a inmate who made the object under abusive conditions, according to CNN.