There is described a pre-creased sheet packaging material for forming a sealed package comprising: a plurality of longitudinal crease lines, which extend along a longitudinal direction; a plurality of transversal crease lines, which extend along a transversal direction; at least one first additional crease line, which extends along a further direction, which is slanted with respect to longitudinal direction and transversal direction; a first joining point, at which longitudinal crease line and a transversal crease line join with respect to one another; and at least one first area, which is bounded by longitudinal crease line, transversal crease line and additional crease line; further direction being incident to first longitudinal crease line and to first transversal crease line in first joining point; at least one of first longitudinal crease line, first transversal crease line and first additional crease line being interrupted at first joining point.