There is described a method for forming a tube (3) of a web (4, 4) of packaging material comprising the steps of advancing the web (4) of packaging material along a web advancement path (P), overlapping a first lateral edge (19) of the web (4, 4′) of packaging material with a second lateral edge (20) of the web (4, 4) of packaging material for obtaining a longitudinal seam portion of the tube (3) and fusing at least an internal outer surface (34) of the first lateral edge (19) and an external outer surface (37) of the second lateral edge (20) with one another for longitudinally sealing the seam portion of the tube (3). The step of fusing comprises at least the substeps of directly heating the external outer surface (37) of the second lateral edge (20) and heating by contact the internal outer surface (34) by establishing contact between the internal outer surface (34) and the directly heated external outer surface (37).