when a key insert corresponding to the coding formed by the arrangement of the flakes is introduced in the rotor the rotation of the key may cause the rotation of the rotor 5 in a first neutral extreme angular position called neutral position such as the u201c stop u201d position two intermediate angular positions called u201c on u201d and u201c accessories u201d positions and a second active extreme angular position opposite to the first neutral extreme angular position called u201c start u201d position the u201c neutral u201d u201c intermediate u201d and u201c start u201d positions follow one another when the rotor 10 is displaced from the u201c neutral u201d position toward the u201c start u201d position after passing through the start position the rotor 10 is resiliently brought back to its intermediate u201c on u201d position by a return spring the locking means includes a bolt 24 movably mounted between a retracted position in which the bolt 24 is kept away from a steering column not represented and a protruding position as illustrated in fig1 in which it blocks the steering column of the motor vehicle for this purpose the cam 12 is intended to be rotatably secured to the rotor so that the rotation of the rotor 10 allows controlling the translation of the bolt 24 the bolt 24 is generally in a protruding position in the steering column when the rotor 10 is in the neutral position the bolt 24 being kept stowed in the retracted position in the other angular positions of the rotor.