Manufacture of flame-retardant regenerated cellulose fibres by adding one or more flame-retardant phosphorus comounds to viscose, extruding the viscose-containing mixture thus obtained into a spinning bath, stretching and after treating the resulting filaments or staple fibres. The flame-retardant fibres are made by using as the flame-retardant agent a combination of compounds of the general formula:PN.sub.x O.sub.yin which x stands for a number between 0.09 and 1.7 and y stands for a number between 1.2 and 0, preferably approaching zero as the values of x increase towards 1.7 (component A), with compounds which are selected from phosphoric acid alkylesters, preferably halogen-containing phosphoric acid alkylesters; cyclophosphazenes, polyphosphazenes; tetrakis-(oxymethyl)-phosphonium chloride or hydroxide or its condensation products with ammonia, oxymethylamine, urea or other amines or amides, vinyl compounds containing halogen, phosphonitrogen or phosphorus; and tris-(1-aziridinyl)-phosphine oxide or N-(oxymethyl)-3-phosphonopropionamide, if desired in further combination with a condensation resin (component B).