The invention relates to a telescopic actuator comprising a cylinder (1) having a closed end (2) and an open end (3) for passing a hollow rod (4) mounted to slide axially in the cylinder. The actuator includes a central support (10) that extends from the closed end of the cylinder inside the rod and that carries obstacles (15) that are movable radially between a setback position leaving the rod free to slide and a projecting position in which the obstacles are engaged in a groove (9) in the rod in order to lock the rod axially relative to the cylinder, the flanks of the obstacles and of the groove being shaped so as to prevent any setting back of the obstacles by movement of the rod, the obstacles being arranged at the ends of spring blades (14) extending longitudinally and arranged to urge the obstacles from the setback position towards the projecting position, the central support carrying controlled means (20) for flexing the blades in order to cause the obstacles to be set back towards the setback position.