A flame retardant composition comprises a phosphorous-containing polymer. The phosphorous-containing polymer can be produced by first reacting a phosphonium compound and a nitrogen-containing compound to produce a precondensate compound and then reacting the precondensate compound with a cross-linking composition. The resulting phosphorous-containing intermediate polymer can then be oxidized to convert at least a portion of the phosphorous atoms in the polymer to a pentavalent state. A textile material comprises a textile substrate and a phosphorous-containing polymer, such as that described above.