A self-stiffened panel of preimpregnated composite. The panel is of a type including a base skin on one side of which are added and placed side-by-side U-shaped stiffeners of which the parts connecting the branches of the U are flattened against the skin, with an interfacing structure between the adjoining branches of two adjacent stiffeners The interfacing structure is a nail having a nail-head comprised of a cord of filling resin. The nail is formed from a band with a rectangular cross-section whose edge, on the skin side, is directly in contact with the adjoining surface of the skin. The nail-head is comprised of two half-nail-heads placed symmetrically in the angle formed between the skin and the nail.