Self-assembly of amphiphilic block copolymers into well-ordered structures has attracted significant interest over the past decade. An especially attractive application of block-copolymer self-assembly is the formation of isoporous membranes. A major problem in this process is the lack of sufficient long-range order and the difficulty of up-scaling due to the time-consuming preparation steps. Here, we report an innovative and simple method to prepare isoporous membranes with nanometre-sized pores. The combination of the industrially well-established membrane formation method by non-solvent-induced phase separation with the self-assembly of a block copolymer is demonstrated. The result is the creation of an integral asymmetric membrane of a block copolymer with a highly ordered thin layer on top of a non-ordered sponge-like layer. This straightforward and very fast one-step procedure for membrane formation is reported for the first time. The developed membrane has the potential for highly selective separation.