Stiffening means are added into an air intake structure of an aircraft engine nacelle, to stiffen this structure, in order to prevent any modification to the shape of the structure, for example following suction of the intake lip inwards the fan duct. The stiffening means pass through the structure diagonally, in other words one of their ends is fixed, for example, in the forward direction and externally, and the other end is fixed in the aft direction and internally. These stiffening means may be added to the existing forward and aft stiffening frames.