A method of manufacturing a single piece multi-spar box made of composite material for aircraft, comprising two panels (11, 12) which are joined together by at least three substantially parallel spars, two of which are outer lateral spars (13a, 13b) and at least one of which is an intermediate spar; the method comprising the steps of: - placing a bottom panel of composite material on a first plate and a top panel of composite material on a second plate; - suspending, above the bottom panel, one or more intermediate spars (14a, 14b) of composite material arranged between elongated inner tools (15a, 15b, 15c) which are each enclosed inside a respective pressurisable impermeable tubular bag; - arranging two outer lateral spars (13a, 13b) of composite material with a C-shaped cross-section inside two respective C-shaped seats (28a, 28b) formed in two respective rigid, lateral, outer tools (27a, 27b), where the two outer lateral spars (13a, 13b) have respective concavities directed towards each other and the two seats (28a, 28b) have respective concavities directed towards each other; - moving the lateral outer tools (27a, 27b) towards each other such that the inner tools wrapped in pressurisable bags are partially inserted inside the C-shaped outer spars (13a, 13b); - simultaneously lowering the inner tools (15a, 15b, 15c) and the lateral outer tools (27a, 27b) so as to bring the bottom bases of the spars (13a, 13b, 14a, 14b) into contact with the bottom panel; - transferring the second plate with the top panel above the spars; - lowering the second plate so as to bring the top panel into contact with top bases of the spars (13a, 13b, 14a, 14b); - locking the two lateral outer tools (27a, 27b) to the first plate and to the second plate, so as to form, together with the plates, a closed mould wherein the lateral outer tools (27a, 27b) are arranged vertically between the first plate and the second plate so as to support the second plate, so that the multi-spar box to be polymerized is closed between the first plate, the second plate and the two lateral outer tools (27a, 27b); - sealing open opposite ends of the tubular bags so that the pressure applied in an autoclave during a following polymerization step causes expansion of the tubular bags, compressing them against bases and cores of the spars; - curing the box in an autoclave by applying a programmed cycle of heat and pressure. - placing a bottom panel of composite material on a first plate and a top panel of composite material on a second plate; - suspending, above the bottom panel, one or more intermediate spars (14a, 14b) of composite material arranged between elongated inner tools (15a, 15b, 15c) which are each enclosed inside a respective pressurisable impermeable tubular bag; - arranging two outer lateral spars (13a, 13b) of composite material with a C-shaped cross-section inside two respective C-shaped seats (28a, 28b) formed in two respective rigid, lateral, outer tools (27a, 27b), where the two outer lateral spars (13a, 13b) have respective concavities directed towards each other and the two seats (28a, 28b) have respective concavities directed towards each other; - moving the lateral outer tools (27a, 27b) towards each other such that the inner tools wrapped in pressurisable bags are partially inserted inside the C-shaped outer spars (13a, 13b); - simultaneously lowering the inner tools (15a, 15b, 15c) and the lateral outer tools (27a, 27b) so as to bring the bottom bases of the spars (13a, 13b, 14a, 14b) into contact with the bottom panel; - transferring the second plate with the top panel above the spars; - lowering the second plate so as to bring the top panel into contact with top bases of the spars (13a, 13b, 14a, 14b); - locking the two lateral outer tools (27a, 27b) to the first plate and to the second plate, so as to form, together with the plates, a closed mould wherein the lateral outer tools (27a, 27b) are arranged vertically between the first plate and the second plate so as to support the second plate, so that the multi-spar box to be polymerized is closed between the first plate, the second plate and the two lateral outer tools (27a, 27b); - sealing open opposite ends of the tubular bags so that the pressure applied in an autoclave during a following polymerization step causes expansion of the tubular bags, compressing them against bases and cores of the spars; - curing the box in an autoclave by applying a programmed cycle of heat and pressure.