A system for detensioning a stay cable by incorporation of, or substitution by, improved filling material, the system comprising: - a formwork in two or more portions suitable for being placed around an area of the stay cable from which a sheath and a cement grout or another initial filler material have been previously removed, if it has these elements, leaving the strands or wires of the stay cable clean, wherein said formwork is arranged in order to be able to inject a filling material with improved features which, once set, provides an area of improved strength and adhesion, such that the cement grout or the another initial filler material, once removed, is placed by the filling material with improved features or, if the stay cable initially lacked a cement grout or another initial filling material, said filling material with improved features is incorporated into the stay cable, - two clamps arranged to be placed on each side of the area of the stay cable with the clean strands or wires, on the area with improved strength and adhesion, on the formwork tube, - two yoke-shaped structures arranged to be placed one on each outer side of each clamp and around the stay cable, - at least two post-tensioning bars or tendons arranged to join together both yoke-shaped structures, - a hydraulic cylinder arranged in each post-tensioning bar or tendon, resting on the yoke-shaped structures, arranged to transfer, when acting on them, the load from the stay cable to the post-tensioning bars or tendons and wherein each hydraulic cylinder can incorporate a tensioning bridge arranged to retain the load transferred to the post-tensioning bars or tendons when acting on them. - a formwork in two or more portions suitable for being placed around an area of the stay cable from which a sheath and a cement grout or another initial filler material have been previously removed, if it has these elements, leaving the strands or wires of the stay cable clean, wherein said formwork is arranged in order to be able to inject a filling material with improved features which, once set, provides an area of improved strength and adhesion, such that the cement grout or the another initial filler material, once removed, is placed by the filling material with improved features or, if the stay cable initially lacked a cement grout or another initial filling material, said filling material with improved features is incorporated into the stay cable, - two clamps arranged to be placed on each side of the area of the stay cable with the clean strands or wires, on the area with improved strength and adhesion, on the formwork tube, - two yoke-shaped structures arranged to be placed one on each outer side of each clamp and around the stay cable, - at least two post-tensioning bars or tendons arranged to join together both yoke-shaped structures, - a hydraulic cylinder arranged in each post-tensioning bar or tendon, resting on the yoke-shaped structures, arranged to transfer, when acting on them, the load from the stay cable to the post-tensioning bars or tendons and wherein each hydraulic cylinder can incorporate a tensioning bridge arranged to retain the load transferred to the post-tensioning bars or tendons when acting on them.