A flame retardant treatment of a lignocellulosic material, which includes: optionally steam exploding the lignocellulosic material, impregnating the optionally steam-exploded lignocellulosic material, in or with an aqueous solution, from 0.5% to 10% of phytic acid and from 1% to 30% of urea, based on the total weight of the aqueous solution, optionally drying of the impregnated lignocellulosic material, until the impregnated lignocellulosic material has a moisture content from 5% to 20% by weight, cooking the impregnated and optionally dried lignocellulosic material, the resulting flame-retarded lignocellulosic material including a phosphorous content originating from the phytic acid from 0.1% to 10% by weight. Also, the resulting flame-retarded lignocellulosic material and the use thereof for manufacturing flame-retarded composite materials based on plant fibres, woven or nonwoven flame-retarded flexible materials based on plant fibres, and particularly textiles, flame-retarded materials based on wood fibres and/or on wood particles, and particularly flame-retarded wood panels.