The invention concerns a method for reinforcing a double-shell structure in which a cellular core (2) is arranged between a first shell (5) and a second shell (6). In the method according to the invention, at least one cavity is formed in the double-shell structure from an opening in the first shell into the cellular core (2) so that the cavity substantially extends through the cellular core (2) to the second shell. Fixative (8) is injected into the cavity. A stiff bar (7) is introduced into the cavity, whereupon excess fixative (8) is allowed to pass out of the cavity. Finally, the fixative left in the cavity is cured.