South Yorkshire Police issued the warning after a spate of burglaries in the Ecclesfield, Chapeltown, High Green and Grenoside areas.
In a Facebook post, the Sheffield North East neighbourhood policing team said: “Over the last week we have seen a number of burglaries in the local area where offenders have gained access via cat flaps, as such it’s a timely reminder to keep these locked at night and to remove any keys or valuables near by to prevent easy access to your homes.
“We currently have a suspect in custody linked to these offences.”
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Cat Flap burglars often push a pole through the opening or letterboxes and hook sets of keys left close to doors, allowing them access to homes.
To pass on information about burglars call South Yorkshire Police on 101 or Crimestoppers, anonymously, on 0800 555111.