Abstract: An arrangement is disclosed for use in conjunction with an antitheft locking fuel line shutoff valve for automotive vehicles which prevents efforts at bypassing or tampering with the valve in attempts to circumvent the locking fuel line shutoff valve. The antitamper arrangement includes a spring-biased fuel line blocking plug disposed within the fuel line and positioned just behind the vehicle fuel pump inlet stem, the blocking plug being restrained from blocking engagement with the fuel pump by means of a flexible cable connected to a rod integral with the blocking plug, the cable passing down the length of the fuel line and anchored at the locking fuel line shutoff valve.