An electric screw actuator system, e.g. for an electrically driven and controlled steering device, comprises a screw actuator, an electric motor (2) engaging the screw actuator through a reduction gear mechanism (3), electronic operating means for operating the screw actuator electrically, and a mechanical back-up system for operating the screw actuator in the absence of electric power e.g. in the case of an electric power failure of in the case the electric power is switched off, maintenance, garage parking etc. The mechanical back-up system comprises a manually controllable operation member (4) and an electromagnetic coupling (6), said coupling disengaging the rotation member from the screw actuator in the case of nominal operation and availability of electric power, and engaging the rotation member with the screw actuator in the case of electric power failure.