The cereal manufacturer said it was “carrying out a full investigation”​ to find out the potential cause of the incident.

A spokeswoman said: “We take food safety extremely seriously and have rigorous quality standards and processes in place in our factories.”​

Discovered the rodent ​

Pauline Henderson, 50, was pouring a bowl of newly-opened cereal for her grandson Toby when she discovered the rodent, according to media reports.

She has since put the box and the mouse in her freezer because she wants environmental health or Kellogg’s to come and look at it.

“I love having my grandson to stay and bought the cereal for him specially when he stayed overnight,”​ she reportedly said. “In the morning I poured some into a bowl for him and noticed something black covered in the cornflakes.​

“I knew something wasn’t right, so I got a spoon and gingerly moved the flakes out of the way. Then I saw the mouse. It was horrible and I felt absolutely sick.”​

The box – bought from a Portsmouth Tesco store – had been stored in the kitchen cupboard and the bag was sealed when Henderson opened it, she claimed.

She feared she would’ve had to rush Toby to hospital if he had consumed the cereal.

Henderson complained to the Tesco supermarket in Portsmouth where she bought the 1kg box of cereal for £1.99.

‘Not taken seriously’ ​

She said she was not initially taken seriously by staff, who found the incident funny.

At time of going to press, Tesco had not responded to FoodManufacture.co.uk’s request for comment.

However, media reports claimed the retailer was ‘concerned’ by the incident and would work with Kellogg to aid its investigation.