A method of making a multilayer panel made of composite material, which method comprises the steps of: a) providing a first fiberglass mat of a predetermined plan size; b) laying a first layer of adhesive resin on the upper face of said first mat; c) providing a three-dimensional honeycomb reinforcement element having a plan size slightly smaller than that of said first mat and laying it over said first mat to embed it at least partially in said first layer of adhesive resin and form a first assembly having a free upper face; f) providing a stone slab of substantially the same size as the first multilayer assembly g) providing a second sheet-like fiberglass mat of substantially the same size as said stone slab and laying it over said slab; h) laying a second layer of adhesive resin on said second mat to impregnate it and promote adhesion thereof to said slab, to define a second multilayer assembly; characterized in that said first multilayer assembly is prepared on a first support plan (P 1) and after step c) is subjected to the following steps: d) overlying said first multilayer assembly with an impermeable sheet sealingly lying over said first support plan (P 1) and defining a first hermetic cavity; e) connecting said first hermetic cavity to a vacuum source for a first predetermined time interval to promote adhesion of said reinforcement element to said first mat; further characterized in that said second multilayer assembly is prepared on a second support plane (P 2) different from the first one (P 1) and after step h) is subjected to the following steps: i) turning said first multilayer assembly upside down and laying it over said second multilayer assembly, with the free face of said first multilayer assembly at least partially embedded in said second layer of adhesive resin to obtain a third multilayer assembly; j) overlaying said third multilayer assembly with the impermeable sheet sealingly lying over the second support plan and defining a second hermetic cavity; k) connecting said second hermetic cavity to a vacuum source for a second predetermined time interval to promote adhesion of said reinforcement element to said first mat and to said second mat and to obtain an intermediate panel or a finished panel; and further characterized in that all the steps a) to k) are carried out at ambient temperature to avoid any heating and warming oven. a) providing a first fiberglass mat of a predetermined plan size; b) laying a first layer of adhesive resin on the upper face of said first mat; c) providing a three-dimensional honeycomb reinforcement element having a plan size slightly smaller than that of said first mat and laying it over said first mat to embed it at least partially in said first layer of adhesive resin and form a first assembly having a free upper face; f) providing a stone slab of substantially the same size as the first multilayer assembly g) providing a second sheet-like fiberglass mat of substantially the same size as said stone slab and laying it over said slab; h) laying a second layer of adhesive resin on said second mat to impregnate it and promote adhesion thereof to said slab, to define a second multilayer assembly; d) overlying said first multilayer assembly with an impermeable sheet sealingly lying over said first support plan (P 1) and defining a first hermetic cavity; e) connecting said first hermetic cavity to a vacuum source for a first predetermined time interval to promote adhesion of said reinforcement element to said first mat; i) turning said first multilayer assembly upside down and laying it over said second multilayer assembly, with the free face of said first multilayer assembly at least partially embedded in said second layer of adhesive resin to obtain a third multilayer assembly; j) overlaying said third multilayer assembly with the impermeable sheet sealingly lying over the second support plan and defining a second hermetic cavity; k) connecting said second hermetic cavity to a vacuum source for a second predetermined time interval to promote adhesion of said reinforcement element to said first mat and to said second mat and to obtain an intermediate panel or a finished panel;