Modular block forming a permanent formwork element for the construction of a portion of a swimming pool, particularly stairs, characterised in that it comprises: - a main surface of substantially rectangular shape, - a first lateral surface (3.1) substantially perpendicular to the main surface, extending along a longitudinal side of the main surface, - a second (3.2) and a third (3.3) lateral surfaces perpendicular to the main surface extending along the transversal sides of the main surface, - a side board extending along a longitudinal side (2.1) of the main surface, one of the walls (5.1) of said side board being located in the extension of the first lateral surface (3.1), said side board forming a protuberance extending from the main surface and delimited by the second (3.2) and the third (3.3) lateral surfaces, this side board having at its end at least one tab, said side board having a profile with a substantially triangular shape or as an alternative, the side board having a rectangular shape, with this side board being formed by a first wall (5.1) extending in the extension of the first lateral surface (3.1) and by a second wall (5.2) forming a non-zero angle, an edge (5.3) carrying the at least one tab extending between the first (5.1) and the second (5.2) wall, and - at least one hollow casting chimney extending from the side opposite that of the side board with respect to the main surface, with this casting chimney being substantially perpendicular to the main surface and having an end that opens into the main surface, - a hollow space located on the opposite side of the side board with respect to the main surface, this hollow space having a complementary shape of the shape of the side board and of the tab, - in such a way that a side board of another modular block can engage with the interior of the hollow space by nesting, the hollow space being delimited by edges (8.1, 8.2) of the second (3.2) and third (3.3) lateral surfaces, said edges (8.1, 8.2) forming a non-zero angle with the transversal edges of the main surface. - a main surface of substantially rectangular shape, - a first lateral surface (3.1) substantially perpendicular to the main surface, extending along a longitudinal side of the main surface, - a second (3.2) and a third (3.3) lateral surfaces perpendicular to the main surface extending along the transversal sides of the main surface, - a side board extending along a longitudinal side (2.1) of the main surface, one of the walls (5.1) of said side board being located in the extension of the first lateral surface (3.1), said side board forming a protuberance extending from the main surface and delimited by the second (3.2) and the third (3.3) lateral surfaces, this side board having at its end at least one tab, said side board having a profile with a substantially triangular shape or as an alternative, the side board having a rectangular shape, with this side board being formed by a first wall (5.1) extending in the extension of the first lateral surface (3.1) and by a second wall (5.2) forming a non-zero angle, an edge (5.3) carrying the at least one tab extending between the first (5.1) and the second (5.2) wall, and - at least one hollow casting chimney extending from the side opposite that of the side board with respect to the main surface, with this casting chimney being substantially perpendicular to the main surface and having an end that opens into the main surface, - a hollow space located on the opposite side of the side board with respect to the main surface, this hollow space having a complementary shape of the shape of the side board and of the tab, - in such a way that a side board of another modular block can engage with the interior of the hollow space by nesting, the hollow space being delimited by edges (8.1, 8.2) of the second (3.2) and third (3.3) lateral surfaces, said edges (8.1, 8.2) forming a non-zero angle with the transversal edges of the main surface.