Barber David Elcock had a close shave when he found a large "furry jellyfish" in a carton of Ribena.
The 37-year-old bought the drink from a city centre shop on Wednesday.
David, who runs Yahweh Hair Salon, in Granby Street, took a gulp from the one-litre juice box and felt something "about the size of a grape" slide down his throat – but it was too late to cough it up.
He poured out a small portion of the contents and out came a blob about the size of a 50p.
When David, from Thurnby Lodge, cut open the carton he found a much larger lump of the mystery substance floating inside.
"I felt sick," he said. "I have no idea what it is, I couldn't even describe it – when a little bit of it came out of the carton it looked like a small furry jellyfish.
"But I couldn't tell you what the big part looks like.
"It's disgusting, it makes me feel ill to think I swallowed part of whatever that is. I felt it slide down my throat and had to swallow hard and immediately knew it wasn't right."
David contacted Ribena, who asked him to send the entire contents of the carton to them.
He also spoke to the owner of the shop where he had bought the drink, who immediately took the product off the shelves.
The shop owner said he had removed any cartons with the same batch number and was sending them back to the supplier.
Images of the mystery ingredient were also sent to Leicester City Council's food safety team.
Team manager David Barclay Rhodes said: "We have had no other reports about this product and this is likely to be an isolated case.
"It is probably due to damage to packaging. This will have allowed mould to form in the drink."
He said that although mouldy food is unpleasant, it is unlikely to be harmful.
A spokesperson for Lucozade Ribena Suntory said: "We produce our products to the highest standards and we take the health and safety of our consumers very seriously.
"We can confirm that the consumer in question has already been in contact with our consumer care team and we are dealing with the enquiry.
"We make it very easy for consumers to get in touch and print our free phone number and address on all our packaging."
