Jonathan Gregory, 27, said he had been wearing the slippers for about a month -- he bought them in January -- when he made the discovery, The Sun reported Sunday.
He said he noticed something rubbing against his foot in one of the slippers and initially thought it was a piece of plastic, but then saw that it was a beak attached to a head.
Gregory cut through the lining of the slipper and found the rest of the bird.
"The worst thing was I was in the middle of making my tea when I found it -- it put me off my pork chops I was so disgusted," he said.
"I've even put my bare feet in those slippers after I've had a bath," Gregory said. "I washed my feet with bleach after I found that bird -- who knows what diseases it has?"
Gregory brought the slippers back to the Primark store where he bought them and was given a refund.
"Primark cares about its customers very much. The company is carrying out an urgent and detailed investigation and apologises to the customer concerned for any distress caused," a Primark spokesman said
The store said, however, that "the last recorded sale of the product in question was in Blackburn was over a year ago."
Gregory said: "I can't believe I've only been given a 4 pound ($5.97) refund, it's outrageous. And now they're trying to say I bought them a year ago. I think I'd remember if I'd had them a whole year."