A modular construction system in which a number of three dimensional construction elements are adjoined to form a modular wall, ceiling or floor assembly. In their pre-assembly condition, each three dimensional construction element is formed from a planar metallic sheet (10) sub-divided by fold lines (12a, 12b) into panels (14, 16, 18) defining a multi-panelled sheet. Each panel lies in a common plane and at least one of the panels is deformable along its fold lines out of said common plane to form an assembled three-dimensional construction element for adjoining to other three-dimensional construction elements. At least one panel is provided with an opening (14a, 16a) dimensioned to allow the passage of a reinforcement or stabilizing material through the assembled three-dimensional construction element.