A process for the production of an oxidized polymer from a tetrakis hydroxyalkyl phosphonium compound comprising NH3 or at least one nitrogen compound comprising at least one NH2 or at least two NH groups, or NH3, comprising the steps of: (a) reacting at least one tetrakis hydroxyalkyl phosphonium compound with NH3 or at least one nitrogen compound in order to obtain a precondensate, wherein the molar ratio of the tetrakis hydroxyakyl phosphonium compound to the nitrogen compound is in the range of 1: (0.05 to 2.0), preferably in the range of 1: (0.5 to 1.5), particularly preferably in the range of 1: (0,65 to 1.2), (b) crosslinking the precondensate obtained in process step (a) with the aid of ammonia to form a crosslinked polymer, (c) oxidizing the crosslinked polymer obtained in step (b) by adding an oxidizing agent to the oxidized polymer, wherein, in step (b), the precondensate from step (a) and the ammonia are each injected by means of a nozzle into a reactor space enclosed by a reactor housing onto a common collision point.