A remotely activated vehicle anti-theft device 10 and system 50 for preventing vehicle theft is provided. The device 10 is moved between unlocked, locked and priority locked states using either a wireless vehicle remote 51 or a mobile phone 52. The mobile phone 52 enjoys priority over the remote control 51 and overrides it when the device is in its priority locked state. It includes a pivotal locking member 17 which is movable between an open position (unlocked state) in which the member is spaced away from a gear 12, permitting it to rotate freely, and a locked position (locked and priority locked states), in which the member 17 engages the gear 12 and prevents rotation thereof, thereby immobilising a vehicle. The device 10 includes an ECU 22, wireless communication module 34, actuator 23, GPS 6 and speed sensor 7 housed in a tamper-proof casing 27.