A pyrotechnic actuator comprises a deformable membrane (24) separating a first chamber (20), which is able to receive pyrotechnic combustion gases, from a second chamber (22) containing a control member (32) displaceable by the membrane (24) during the operation of the actuator. In the inoperative state, the membrane (24) is prestressed counter to its deformation controlled by the intake of gases into the first chamber (20). In this way the functional travel is increased, which e.g. makes it possible to open or close a circuit with a larger cross-section without increasing the actuator size.