Abstract: A liner (1) intended as inner packing in a container (11) for holding a fluid, comprising an upper (2) and a lower (3) rectangular web of essentially the same length and width, which are made of a flexible material and which are joined (4) to each other along portions of their outer edges (8, 9, 34), and at least one sealable opening (5, 6) in one of the webs (3) for filling the liner (1) with said fluid and then emptying the liner of said fluid in the container (11). The novelty of the invention is that the liner integrally comprises a pair of flap members (41, 42) at one of the outer edges (33) between the webs (2, 3), that one flap member (41) has been folded over the upper web (2), that the corner portions (13) of the liner adjacent to the essentially unfolded flap member (42) have been folded towards the center over the upper web (2), and that the liner (1) has been folded symmetrically at least twice towards the symmetry line (7) over the upper web (2).