Primary support beam for slab formwork or the like, having a load-bearing upper surface designed to bear a load, extending in a first longitudinal direction (I-I), comprising a first segment and a second segment extending respectively in second (II-II) and third (III-III) longitudinal directions, said first and second segments being arranged so as to slide with respect to one another in the first longitudinal direction (I-I) by rail means such that they can be displaced between a retracted position in which one segment (2, 3) is more overlapped by the other, and at least one extended position in which one segment (2, 3) is less overlapped by the other than in the retracted position, in which: - on each side on the first and second segments, there are provided respective lateral receiving channels (2a, 2b, 3a, 3b) for receiving a secondary support beam, - each lateral receiving channel (2a, 2b, 3a, 3b) comprises: • a lateral flange (20a, 20b, 30a, 30b) designed to receive, bearing vertically, the connecting structure of a secondary support beam, • a lateral rim (23a, 23b, 33a, 33b) shaped so as to oppose any axial displacement outside the first longitudinal direction (I-I) of the connecting structure of the secondary support beam, extending from and away from the lateral flange (20a, 20b, 30a, 30b) towards the load-bearing upper surface, characterized in that: - said first and second segments are arranged and held so as to always be co-linear with respect to one another in the first longitudinal direction (I-I) by the rail means, - the respective lateral receiving channels (2a, 2b, 3a, 3b) of the first and second sections slide one inside the other and have a permanent overlap (r) with respect to one another in the first longitudinal direction (I-I) whatever the relative positions of the first and second segments with respect to one another, sliding between the retracted position and said at least one extended position. - on each side on the first and second segments, there are provided respective lateral receiving channels (2a, 2b, 3a, 3b) for receiving a secondary support beam, - each lateral receiving channel (2a, 2b, 3a, 3b) comprises: • a lateral flange (20a, 20b, 30a, 30b) designed to receive, bearing vertically, the connecting structure of a secondary support beam, • a lateral rim (23a, 23b, 33a, 33b) shaped so as to oppose any axial displacement outside the first longitudinal direction (I-I) of the connecting structure of the secondary support beam, extending from and away from the lateral flange (20a, 20b, 30a, 30b) towards the load-bearing upper surface, • a lateral flange (20a, 20b, 30a, 30b) designed to receive, bearing vertically, the connecting structure of a secondary support beam, • a lateral rim (23a, 23b, 33a, 33b) shaped so as to oppose any axial displacement outside the first longitudinal direction (I-I) of the connecting structure of the secondary support beam, extending from and away from the lateral flange (20a, 20b, 30a, 30b) towards the load-bearing upper surface, - said first and second segments are arranged and held so as to always be co-linear with respect to one another in the first longitudinal direction (I-I) by the rail means, - the respective lateral receiving channels (2a, 2b, 3a, 3b) of the first and second sections slide one inside the other and have a permanent overlap (r) with respect to one another in the first longitudinal direction (I-I) whatever the relative positions of the first and second segments with respect to one another, sliding between the retracted position and said at least one extended position.