Abstract: The present application relates to a protecting sleeve for shielding and reinforcing a bicycle lock mechanism, which sleeve can conveniently be applied to many existing bicycle locks. The sleeve comprises an elongate body having a central bore sized to receive a cylindrical portion of a lock in which the lock mechanism is located. It has been found that this area of the lock is the weakest and the lock can be defeated by breaking the locking mechanism. The protecting sleeve includes intermediate its length a channel passage through which a locking member can pass for releasably engaging the locking mechanism. The protecting sleeve is preferably made of a high strength material, such as an aircraft aluminum alloy material which also acts as a thermal moderator.