Mark Hunter, 44, was pouring himself cereal when the rodent rolled into the bowl.
However, it wasn't until he started eating his breakfast that he realised what was in the milk.
Hunter told The Mirror: "I was horrified because I'd already had one mouthful and was about to have a second.
"I saw this dark shape appear in the milk, but I didn't realise what it was until I fished it out with my spoon."
A Kellogg's spokesman stated that the food manufacturing company is investigating where the mouse came from, stressing that it was an "extremely rare" case.
He said: "We are devastated and will be investigating fully."
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