The invention provides a removable security device (100) which in a preferred embodiment is inserted into an existing cigar/cigarette lighter socket of a vehicle and acts as an anti-theft system. The security device (100) transmits an authorisation code through the electrical network of the vehicle. The code is detected by enabling devices (200) which are connected in series with components (300) of the vehicle, e.g. the starter motor and/or fuel pump. When the or each enabling device (200) receives a bona fide authorisation code it switches on thereby allowing the component to be operated. As the enabling devices (200) are small they can be fitted into components at manufacture or fitted to existing components. The system has a very low power requirement and therefore does not need cooling. It has safety features which ensure that if the vehicle stalls the components are not disabled. An advantage of the invention is that no major modifications need to be made to a vehicle.
