A shelter having a predetermined standard length and a longitudinal axis, and comprising a parallelepiped-shaped central element or module and two parallelepiped-shaped end elements or modules (4a, 4b), which are aligned with the central module along the longitudinal axis and are rigidly connected to the central module at respective opposite axial ends of the central module; the central module has a first length shorter than said standard length, and the end modules (4a, 4b) have respective second and third lengths, which are equal or different, but are complementary lengths so that their sum is equal to the difference between the standard length and the first length; the shelter comprising a plurality of ISO-standard corner blocks defining respective external vertices of the shelter; wherein the central module comprises a first framework comprising a plurality of bars (8, 9a, 9b, 10a, 10b) and is closed by panels (15, 16, 17, 18) carried by the first framework, and each end module (4a; 4b) comprises a respective second framework (6a; 6b), structurally independent from the first framework and comprising a plurality of tubes (20a, 20b, 21a, 21b, 22a, 22b) connected to one another by means of said ISO-standard corner blocks; wherein the panels (15, 16, 17, 18) are arranged simply in contact with the framework from the inside and are fixed in a removable manner, by means of screws, rivets or the like, to the respective bars (8, 9a, 9b, 10a, 10b) of the framework; wherein the first framework comprises two first frames (7a, 7b), which are identical, are right-parallelogram-shaped, are aligned to and spaced apart from one another along said longitudinal axis and lie on respective planes perpendicular to the longitudinal axis; each first frame (7a, 7b) comprises a pair of transversal bars (9a, 9b), and a pair of vertical bars (10a, 10b) arranged at right angles to the transversal bars (9a, 9b); wherein the first framework further comprises four longitudinal bars, each of which is parallel to said longitudinal axis and extends from an edge of a first frame (7a; 7b) to a corresponding edge of the other first frame (7b; 7a); wherein, at each axial end, each longitudinal bar is rigidly connected to an end of a respective transversal bar (9a; 9b) and to an end of a respective vertical bar (10a; 10b) by means of a corner joint, which defines, with the other corner joints, the eight vertices of the central module; wherein each of said bars (8, 9a, 9b, 10a, 10b) is a metal L-shaped bar comprising two flat flanges having substantially the same width and defining therebetween a right dihedral facing the longitudinal axis; wherein each corner joint is defined by three flat plates, each of which is integral with and perpendicular to the other two plates and is rigidly connected, by means of screws, rivets or other analogous fastening means, to the flanges of two of the three bars (8, 9a, 9b, 10a, 10b) that converge to the corner joint; wherein the form, the number and the arrangement of the respective components of said second frameworks (6a; 6b) are the same and can only differ with regard to their longitudinal dimension, i.e. the length, measured parallel to said longitudinal axis, of the respective end module (4a, 4b); wherein each second framework (6a; 6b) comprises a second frame (19a; 19b), which faces a respective first frame (7a; 7b), has a form and dimensions identical to those of the first frames (7a, 7b), lies on a plane perpendicular to the longitudinal axis, and comprises a pair of transversal tubes (20a, 20b) and a pair of vertical tubes (21a, 21b) which are arranged at right angles to the transversal tubes (20a, 20b); each second framework (6a; 6b) further comprises four longitudinal tubes (22a, 22b), each of which is parallel to the longitudinal axis, extends from an edge of a second frame (19a; 19b) to an edge of a facing first frame (7a; 7b), and is aligned to a corresponding longitudinal bar; wherein said transversal tubes (20a, 20b), vertical tubes (21a, 21b) and longitudinal tubes (22a, 22b) are hollow metal sections with a square cross section, each longitudinal tube (22a; 22b) being connected, at one axial end, to an end of a transversal tube (20a; 20b) and an end of a vertical tube (21a; 21b) by means of a said ISO-standard corner block; wherein, at its axial end facing the first frame (7a; 7b), each longitudinal tube (22a; 22b) is rigidly connected to a respective said corner joint by means of a plug-and-socket coupling; and wherein each corner joint has a projecting element, which projects from the respective first frame (7a; 7b) toward the second frame (19a; 19b) facing the first frame (7a; 7b) and is fitted inside an axial end of a respective said longitudinal tube (22a; 22b) to define the respective said plug-and-socket coupling.