Abstract: A removable cap for fuel tanks of vehicles includes a gripping portion and a locking portion, operatively set between which is a key-operated locking device. In the closing condition of the device, a rotation movement imparted manually upon the gripping portion is not transferred to the locking portion, while in the opening condition of the device a rotation movement imparted manually upon the first part is transferred to the clamping portion. In the closing condition at least one component of the locking device is rotatable together with the gripping portion, and switching with the key of the locking device between the conditions of closing and opening causes an angular movement of the component with respect to the gripping portion, between a first pre-defined angular position and a second pre-defined angular position.