The maggot was reportedly found by 22-year-old Nick Todd as he bit into the product bought from a London store on Wednesday (December 3).
Todd said he was horrified, felt physically sick and was put off purchasing Kellogg products for life, according to media reports.
“It was sitting on top of the bar, wriggling around a bit,” he claimed.
“I can’t believe there was something in there, let alone it was still alive, I was nearly sick.”
A spokeswoman for Kellogg said the cereal and snack manufacturer would launch a full investigation.
“We take all complaints of this nature very seriously,” she said. “This was reported to us on Wednesday and when we receive the specimen we will carry out a full and thorough investigation.
“It’s extremely unlikely the specimen entered the food in our factory due to the high temperatures in the cooking process.”