An integrally blow-moulded bag-in-container has as integrally blow-moulded bag-in-container wherein the same polymer is in contact on either side of the interface between the inner and outer layers. A preform for blow-moulding a bag-in-container has an inner layer and an outer layer, wherein the preform forms a two-layer container upon blow-moulding, and wherein the thus obtained inner layer of the container releases from the thus obtained outer layer upon introduction of a gas at a point of interface between the two layers. The inner and outer layers are of the same material.