Fifty-year-old Pauline Henderson was only trying to serve breakfast to her grandson, Toby, when she noticed something much different than a prize fall from a freshly-unsealed box of cereal.
"In the morning I poured some into a bowl for him and noticed something black covered in the corkflakes," Henderson told the Daily Mirror.
"Then I saw the mouse. It was horrible and I felt absolutely sick ... Goodness knows how long the mouse was in the box or if it was dead or alive when it went in."
Kellogg's in investigating the incident, and says this is a first for the popular cereal manufacturer.
"We are carrying out a full investigation to identify if and how this occurred as it hasn't happened before," a spokesperson said, adding, "We take food safety extremely seriously and carry out regular quality checks in our factories."
Kellogg's offered to replace the box, but Henderson said she'll pass.
"I spoke to Kellogg's and they asked me if I wanted another box and I said 'no I do not' ... It's put me off buying any Crunchy Nut ever again."