A preform for an integrally blow-moulded bag-in-container uses an inner layer and an outer layer, wherein the preform forms a two-layer container upon blow-moulding, and wherein the 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. At least one of the inner and outer layers includes at least one additive allowing both inner and outer layers to reach their respective blow-moulding temperatures substantially simultaneously.