A method of manufacturing reinforced panels of composite material, comprising a skin panel and a plurality of longitudinally elongated stringers rigidly and integrally applied to one face of the skin panel, the method comprising the following steps: a1) providing a plurality of stringers of thermosetting fiber-reinforced polymerizable composite material, each stringer having a cross section with a concave portion and two opposite lateral flanges (16, 17); a2) providing a skin panel of fiber-reinforced thermosetting composite polymerizable or polymerized material; a3) providing a plurality of longitudinally elongated inserts having a solid or not hollow cross-section with a convex surface substantially congruent with the concave portion of the stringer and a substantially flat surface, wherein the material of the inserts has coefficients of expansion and of thermal shrinkage higher than coefficients of expansion and thermal shrinkage of the thermosetting composite material; b) assembling the panel by placing the flanges (16, 17) of each stringer against a face of the skin panel, with a respective insert interposed between the concave portion and the face of the skin panel, in such a way that the stringer and the face of the skin panel together define a respective plurality of elongate cavities having a closed loop cross-section which receive the inserts and are congruent thereto; c) covering the panel with a curing vacuum bag and applying vacuum to the bag; d) applying heat and pressure to the panel in an autoclave so as to cure the polymerizable composite material, in such a way that each insert undergoes a controlled thermal expansion and exerts a pressure against inner walls of the cavity in which it is received, which pressure reacts to a compressive action resulting from the external pressure of the autoclave and the vacuum applied to the bag, resulting in compaction of the stringers; e) allowing the inserts to cool down, whereby the inserts undergo a thermal shrinkage of higher magnitude than the thermal shrinkage undergone by the cured panel; f) removing the inserts out of the cavities; characterized in that the longitudinally elongated inserts are made of rubber material having coefficients of expansion and of thermal shrinkage higher than coefficients of expansion and thermal shrinkage of the thermosetting composite material, and that in step f), the longitudinally elongated inserts are removed by extracting the inserts longitudinally out of the cavities. a1) providing a plurality of stringers of thermosetting fiber-reinforced polymerizable composite material, each stringer having a cross section with a concave portion and two opposite lateral flanges (16, 17); a2) providing a skin panel of fiber-reinforced thermosetting composite polymerizable or polymerized material; a3) providing a plurality of longitudinally elongated inserts having a solid or not hollow cross-section with a convex surface substantially congruent with the concave portion of the stringer and a substantially flat surface, wherein the material of the inserts has coefficients of expansion and of thermal shrinkage higher than coefficients of expansion and thermal shrinkage of the thermosetting composite material; b) assembling the panel by placing the flanges (16, 17) of each stringer against a face of the skin panel, with a respective insert interposed between the concave portion and the face of the skin panel, in such a way that the stringer and the face of the skin panel together define a respective plurality of elongate cavities having a closed loop cross-section which receive the inserts and are congruent thereto; c) covering the panel with a curing vacuum bag and applying vacuum to the bag; d) applying heat and pressure to the panel in an autoclave so as to cure the polymerizable composite material, in such a way that each insert undergoes a controlled thermal expansion and exerts a pressure against inner walls of the cavity in which it is received, which pressure reacts to a compressive action resulting from the external pressure of the autoclave and the vacuum applied to the bag, resulting in compaction of the stringers; e) allowing the inserts to cool down, whereby the inserts undergo a thermal shrinkage of higher magnitude than the thermal shrinkage undergone by the cured panel; f) removing the inserts out of the cavities; the longitudinally elongated inserts are made of rubber material having coefficients of expansion and of thermal shrinkage higher than coefficients of expansion and thermal shrinkage of the thermosetting composite material, and that in step f), the longitudinally elongated inserts are removed by extracting the inserts longitudinally out of the cavities.