Police believe teenage pranksters are hacking into the wireless frequency of a US Burger King drive-through speaker to tell potential customers they are too fat for fast food.

Policeman Gerry Scherlink said the pranksters told one customer who had just placed an order: "You don't need a couple of Whoppers. You are too fat. Pull ahead."

The offenders are reportedly tapping into the wireless frequency at the restaurant in Troy, Michigan. Police believe the culprits are watching and broadcasting from close range.

Officer Scherlinck said the men are telling customers who order a Coca-Cola that, "We don't have Coke." And when the customer asks what they do have, the hacker would say: "We don't have anything. Pull ahead."