Abstract: A self-locking, high-pressure stopper is disclosed. Also disclosed is a method of using the stopper. The stopper allows a branch pipe to be plugged from above ground using an elongated insertion tool. The self-locking stopper of the present invention includes a body having O-ring seals that is inserted into the branch pipe that is to be plugged. The self-locking stopper has outwardly biased locking tabs that are provided to engage a shoulder or step in the branching pipe in order to prevent the stopper from being pulled out, or pushed out by fluid pressure, once it is in place. The stopper system also includes a flanged insertion pipe used for inserting the stopper into the branch pipe. The stopper also has an adapter for engagement by an insertion tool that allows a user to insert the stopper from above ground.