A stopper for application to a container of the type with a base, an emptying duct, a lid, mounted on the base to pivot between an open and a closed position, a hinge with elastic return which holds the lid in the open position thereof and locking means, for locking the lid on the base. The stopper comprises positive unlocking means for the lid from the base, which are manually operated to provide a force to disengage the holding means. The base is a unitary construction and made from plastic material, the lid is made from the same plastic material and the leaf of the hinge is in an elastomeric plastic material.