A vehicle anti-theft system, includes in a passive relay enabling an electronic label (2) usually identified when proximate to a transponder (1) to be offset. The electronic label (2) is integrated in a reinforced casing. Communication between the label and the transponder (1) is set up via an antenna (4) proximate the transponder (1), an electric cable and a second antenna (5) proximate the label. The cable includes an electromechanical switch enabling a control switch (8) to interrupt or set up the radio link. The invention therefore provides a novel anti-theft system for which the installation is entirely reversible and having no damaging effect on the vehicle since it does not require disconnecting a vehicle cable and entails no splicing.