Described herein is a process for producing flexible polyurethane foams, in which (a) polyisocyanate prepolymer, is mixed with (b) polymeric compounds having groups reactive toward isocyanates, (c) optionally chain extenders and/or crosslinkers, (d) catalyst, (e) blowing agent comprising water and (f) optionally additives to give a reaction mixture and reacted to give a flexible polyurethane foam.