Abstract: A system 5 for securely blocking relative translational movement between a cylindrical shaft and an element 112, includes a blocking element 50 in which an immobilization element 55 is arranged, the immobilization element being associated with a viscoelastic fluid 8 disposed between the blocking element 50 and the immobilization element 55 in such a way that translational movement is blocked when the viscoelastic fluid 8 is pressurized in the blocking cylinder and translational movement is possible when the viscoelastic fluid 8 is no longer pressurized. An actuating cylinder equipped with such a secure blocking system is also described.