Cameron Murray, 11, says that he had already eaten a large chunk of the Cadbury Dairy Milk bar before spotting the insect imprinted on the bottom of it.
The youngster, of Barlby, near Selby, immediately ran to tell his mother, Sarah, 37 – complainin
g he felt "sick" after eating half of the bar that he had been saving from his Christmas selection box.
He said: "When I saw the bug I thought I was going to be sick. I was just hoping that I had not eaten any of it."
His mother said she has reported the incident to consumer watchdogs.
She said: "I was on the phone when he came into the room and said there was a bug in his chocolate bar.
"He started drinking loads of water. He told me he had eaten a bug and I thought he was having me on. But when I took the bar off him I saw it. It was imprinted on the underside of the chocolate bar.
"It looked like a moth you would see in a display cabinet, with its wings outstretched. It is about the size of a thumbnail. I thought it was disgusting. It really is horrible thinking bugs are in your food."
Cameron had kept the selection box, a Christmas gift, in the fridge and had saved his favourite Dairy Milk as a treat.
A Cadbury spokesman said: "We are sorry to hear that Cameron has experienced this as it sounds unpleasant for him.
"We would ask his mother to return the bar to Cadbury as soon as possible so that we can carry out a full investigation and we will keep them fully informed."
But there had been no direct complaint by Cameron's family to the company and they had not seen the offending product, the spokesman said.
