A horrified mum found 15 dead files on the fries in her young son’s McDonald’s Happy Meal.
Hannah Mount, 28, bought the food for Joshua, six, at a drive-thru restaurant.
She spotted the insects after autistic Joshua had finished eating.
Hannah said: “My son is fussy with his food.
“He told me he had finished and didn’t want any more as one was burnt.
“He passed me his Happy Meal and there were about four chips left. I was going to eat them — then saw the burnt bit was in fact a bundle of dead flies. It was disgusting.”
Hannah, of Padiham, Lancs, stopped for the food in Burnley with a pal and the friend’s son.
The mum, who also had her daughter, Hailey, two, with her said she thought the flies may have been in the container in which the grub was served.
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Hannah, a full-time carer for Joshua, who has muscular dystrophy, said: “I think they were in the box as my son empties everything into the box.
“I doubt someone would do this to anyone on purpose. It’s just a mistake.”
Victor Arciniega, who runs the franchise with wife Karen, told Hannah he was “shocked and disappointed” after she told him of her fly discovery.
He promised to “get to the bottom” of it.
McDonald’s confirmed it was investigating.