Vehicle security system for vehicle fleets

A vehicle security system for securing a vehicle equipped to determine that a valid radio frequency identification RFID tag (14) is proximate to the vehicle when there is an attempt to start the vehicle. The system includes an authorization device (24) configured to determine that vehicle operation has been authorized, and an interference device (30) configured to receive an authorization signal from the authorization device. The interference device e.g. a jamming device is configured to interfere with the determination that a valid radio frequency identification tag is proximate to the vehicle when the authorization signal does not indicate that vehicle operation has been authorized. The system is readily installed in a vehicle as no electrical connections are required, except for an optional power connection.