Liquid hardeners for hardening curing polymer resins, in particular epoxy resins, including a) cyanamide; b) at least one urea derivative of the formula or formula wherein the following applies to the radicals, in each case simultaneously or independently of one another: R 1, R 2 = simultaneously or independently of one another hydrogen, C1- to C15-alkyl, C3- to C15-cycloalkyl or together form a ring of C3- to C10-alkyls; R 3 = hydrogen, C1- to C15-alkyl, C3- to C15-cycloalkyl, aryl, arylalkyl, - NHC NR 1 R 2 -substituted C1- to C15-alkyl, - NHC NR 1 R 2 -substituted C3- to C15-cycloalkyl, - NHC NR 1 R 2 -substituted aryl or - NHC NR 1 R 2 -substituted arylalkyl; R 4, R 5, R 6, R 7, R 8 = simultaneously or independently of one another hydrogen, halogen, C1- to C15-alkyl, C3- to C15-cycloalkyl, aryl, arylalkyl, - CF 3,-NHC NR 1 R 2, - NHC NR 1 R 2 -substituted C1- to C15-alkyl, - NHC NR 1 R 2 -substituted aryl or - NHC NR 1 R 2 -substituted arylalkyl; and c) at least one stabiliser selected from the group comprising inorganic or organic acids. a) cyanamide; b) at least one urea derivative of the formula or formula wherein the following applies to the radicals, in each case simultaneously or independently of one another: R 1, R 2 = simultaneously or independently of one another hydrogen, C1- to C15-alkyl, C3- to C15-cycloalkyl or together form a ring of C3- to C10-alkyls; R 3 = hydrogen, C1- to C15-alkyl, C3- to C15-cycloalkyl, aryl, arylalkyl, - NHC NR 1 R 2 -substituted C1- to C15-alkyl, - NHC NR 1 R 2 -substituted C3- to C15-cycloalkyl, - NHC NR 1 R 2 -substituted aryl or - NHC NR 1 R 2 -substituted arylalkyl; R 4, R 5, R 6, R 7, R 8 = simultaneously or independently of one another hydrogen, halogen, C1- to C15-alkyl, C3- to C15-cycloalkyl, aryl, arylalkyl, - CF 3,-NHC NR 1 R 2, - NHC NR 1 R 2 -substituted C1- to C15-alkyl, - NHC NR 1 R 2 -substituted aryl or - NHC NR 1 R 2 -substituted arylalkyl; and c) at least one stabiliser selected from the group comprising inorganic or organic acids. R 1, R 2 = simultaneously or independently of one another hydrogen, C1- to C15-alkyl, C3- to C15-cycloalkyl or together form a ring of C3- to C10-alkyls; R 3 = hydrogen, C1- to C15-alkyl, C3- to C15-cycloalkyl, aryl, arylalkyl, - NHC NR 1 R 2 -substituted C1- to C15-alkyl, - NHC NR 1 R 2 -substituted C3- to C15-cycloalkyl, - NHC NR 1 R 2 -substituted aryl or - NHC NR 1 R 2 -substituted arylalkyl; - NHC NR 1 R 2 -substituted C1- to C15-alkyl, - NHC NR 1 R 2 -substituted C3- to C15-cycloalkyl, - NHC NR 1 R 2 -substituted aryl or - NHC NR 1 R 2 -substituted arylalkyl; R 4, R 5, R 6, R 7, R 8 = simultaneously or independently of one another hydrogen, halogen, C1- to C15-alkyl, C3- to C15-cycloalkyl, aryl, arylalkyl, - CF 3,-NHC NR 1 R 2, - NHC NR 1 R 2 -substituted C1- to C15-alkyl, - NHC NR 1 R 2 -substituted aryl or - NHC NR 1 R 2 -substituted arylalkyl; and c) at least one stabiliser selected from the group comprising inorganic or organic acids. - CF 3,-NHC NR 1 R 2, - NHC NR 1 R 2 -substituted C1- to C15-alkyl, - NHC NR 1 R 2 -substituted aryl or - NHC NR 1 R 2 -substituted arylalkyl; and c) at least one stabiliser selected from the group comprising inorganic or organic acids.