A method for reinforcing a panel, flat or preformed in three dimensions, according to its thickness E: - a step a) of providing a reinforcing element which is elongated, flexible, with two longitudinal ends (10, 11) having a length L strictly greater than or equal to twice the thickness E of the panel at the point of insertion, - a step b) of positioning and holding said reinforcing element on one side of the panel, called the insertion side (Ci) - a step c) of gripping the reinforcing element, on the insertion side in an intermediate position between the two longitudinal ends of the reinforcing element, - an insertion step d) wherein said reinforcing element gripped in the intermediate position is pulled through a bore passing through the panel, by folding the reinforcing element back on itself, and to a final position (Pf) wherein the two longitudinal ends (10, 11) project from the bore on the insertion side (Ci) or else are flush with the wall of the panel on the insertion side, a loop of the reinforcing element projecting from the bore on the side of the panel opposite to the insertion side (Ci), or else being provided in a position in the vicinity of the outer wall of the panel on the side of the panel opposite to the insertion side configured so that a section of the loop divides the reinforcing element into two length sections (15, 16) each extending through the bore, positioning the ends (151, 161) of the two length sections (15, 16) flush with the outer wall of the panel on the side opposite to the insertion side (Ci), and wherein the reinforcement method has a step of forming the bore, prior to the insertion step d), wherein said bore is drilled into the panel by means of a drilling tool comprising a cylindrical body provided at its base with a circular cutting edge, and wherein the bore is formed by a movement combining an advance along the axis of the cylindrical body and a rotation around the axis of the cylindrical body, generating a drill core resulting from the drilling of the bore, internal to the cylindrical body. - a step a) of providing a reinforcing element which is elongated, flexible, with two longitudinal ends (10, 11) having a length L strictly greater than or equal to twice the thickness E of the panel at the point of insertion, - a step b) of positioning and holding said reinforcing element on one side of the panel, called the insertion side (Ci) - a step c) of gripping the reinforcing element, on the insertion side in an intermediate position between the two longitudinal ends of the reinforcing element, - an insertion step d) wherein said reinforcing element gripped in the intermediate position is pulled through a bore passing through the panel, by folding the reinforcing element back on itself, and to a final position (Pf) wherein the two longitudinal ends (10, 11) project from the bore on the insertion side (Ci) or else are flush with the wall of the panel on the insertion side, a loop of the reinforcing element projecting from the bore on the side of the panel opposite to the insertion side (Ci), or else being provided in a position in the vicinity of the outer wall of the panel on the side of the panel opposite to the insertion side configured so that a section of the loop divides the reinforcing element into two length sections (15, 16) each extending through the bore, positioning the ends (151, 161) of the two length sections (15, 16) flush with the outer wall of the panel on the side opposite to the insertion side (Ci),