The present invention relates to a liquid composition comprising a monomer, a (meth)acrylic polymer and at least one flame-retardant substance (FD1) chosen from a phosphorus-based additive. In particular the present invention it relates to a liquid composition comprising a monomer, a (meth)acrylic polymer and at least one flame-retardant substance (FD1) chosen from a phosphorus-based additive that can be used as a syrup and especially as a syrup for impregnation and for the preparation for improving the fire resistance of a thermoplastic polymer or matrix obtained after polymerization of the syrup. Also concerned is a thermoplastic material obtained after polymerization of the liquid composition or syrup. The invention also relates to a process for manufacturing such a liquid composition or syrup. The invention also relates to a process for impregnating a fibrous substrate of long fibers with said liquid composition or syrup. The invention also relates to a fibrous substrate preimpregnated with said liquid composition or syrup which is useful for manufacturing composite parts. The present invention also relates to a process for manufacturing mechanical parts or structural elements made of composite material and to mechanical parts or structural elements made of composite material obtained via a process using such a liquid composition.