A method of manufacturing an elongate structural element of composite material elongated in a longitudinal direction and having at least one web and at least one flange (11, 12) and a nodal area extending along a junction between the web and the flange, the method comprising the steps of: - providing at least three components (15, 16, 27) of composite material, at least one of which is uncured, wherein the three components include a first component having at least a flange portion, a web portion and at least one first radiused portion joining the flange portion to the web portion, a second component having at least a flange portion, a web portion and at least one second radiused portion joining the flange portion to the web portion, at least one third component having a flat base; - preassembling the first and second components by bringing the web portion of the first component against the web portion of the second component, in such a way that the flange portions (151, 161) of the first and second components extend in opposite directions and lie in the same plane, and the radiused portions (155, 165) of the first and second components are adjacent and together form a longitudinally extending recess; - providing at least one removable resilient insert made of pre-cured silicone rubber of the vinyl-methyl-polysiloxane type belonging to siloxane group Q according to the ISO 1629 standard, said insert having a substantially triangular or pseudo-triangular cross section with a flat base and two other sides (22, 23) converging into a ridge, wherein said other two sides (22, 23) have shapes mating with the respective radiused portions (155, 165) of the first and the second components; - applying the insert along the recess by inserting the ridge between the radiused portions (155, 165) placing the flat base in the plane in which the flange portions (151, 161) of the first and second component (15, 16) lie; - applying the at least one third component (27, 28) with the flat base against the portions of the flange (151, 161) of the first and second component (15, 16) and the flat base of the insert; - after vacuum bagging, applying at least one programmed cycle of temperature and pressure in an autoclave to cure the uncured components; - upon completion of the curing step, longitudinally extracting said at least one insert from the elongate structural element, thus obtaining a monolithic elongate structural element which forms at least one cavity extending longitudinally through the nodal area and having a shape corresponding to the shape of the insert. - providing at least three components (15, 16, 27) of composite material, at least one of which is uncured, wherein the three components include a first component having at least a flange portion, a web portion and at least one first radiused portion joining the flange portion to the web portion, a second component having at least a flange portion, a web portion and at least one second radiused portion joining the flange portion to the web portion, at least one third component having a flat base; - preassembling the first and second components by bringing the web portion of the first component against the web portion of the second component, in such a way that the flange portions (151, 161) of the first and second components extend in opposite directions and lie in the same plane, and the radiused portions (155, 165) of the first and second components are adjacent and together form a longitudinally extending recess; - providing at least one removable resilient insert made of pre-cured silicone rubber of the vinyl-methyl-polysiloxane type belonging to siloxane group Q according to the ISO 1629 standard, said insert having a substantially triangular or pseudo-triangular cross section with a flat base and two other sides (22, 23) converging into a ridge, wherein said other two sides (22, 23) have shapes mating with the respective radiused portions (155, 165) of the first and the second components; - applying the insert along the recess by inserting the ridge between the radiused portions (155, 165) placing the flat base in the plane in which the flange portions (151, 161) of the first and second component (15, 16) lie; - applying the at least one third component (27, 28) with the flat base against the portions of the flange (151, 161) of the first and second component (15, 16) and the flat base of the insert; - after vacuum bagging, applying at least one programmed cycle of temperature and pressure in an autoclave to cure the uncured components; - upon completion of the curing step, longitudinally extracting said at least one insert from the elongate structural element, thus obtaining a monolithic elongate structural element which forms at least one cavity extending longitudinally through the nodal area and having a shape corresponding to the shape of the insert.