The invention relates to a key, in particular for an automobile, which comprises:—a casing (3) with an upper half shell (3a) defining a lid and a lower half shell (3b) defining a casing bottom;—an insert (5, 7) pivotally mounted relative to the casing (3) and capable of movement between a rest position in which the insert (5, 7) is stowed in said casing (3) and a use position in which the insert (5, 7) is extended relative to said casing (3);—an insert (5, 7) extension device mounted in the casing (3) and including: a push button (19) axially received in a cavity (21) combined with the upper half shell (3a) and protruding from the upper half shell (3a) so as to be actuated by a user; and an elastic return member (23) for the insert (5, 7), having a first end (23a) connected to the insert (5, 7) for pivotally urging the insert (5, 7) into the use position when the push button (19) is actuated. According to the invention, the push button (19) includes a means for the rotational blocking of the push button (19) relative to the casing that defines an axial-translation guide for the push button (19), and said return member (23) is attached to the push button (19) by a second end (23b). The invention further relates to an extension module for the insert (5, 7) of such a key.