The pouch (10) comprises a flexible bag (12) having an opening (12A) with its edge fastened to two opposite walls (20A, 20B) of a reinforcing collar (18) projecting out from the bag, the pouch being suitable for adopting a flat configuration in which the collar is folded along a main fold line (17) connecting together its walls so that said walls are situated one against the other, and a utilization configuration in which the walls form a channel holding the opening open. The main fold line (17) presents an end portion (17A) opposite from the opening that projects laterally outwards, such that in the utilization configuration, the collar presents a nose defined by said end portion.
