An unlockable non-return valve reduces the tendency of the poppet piston to jam by providing a first insert that has a guide diameter for the poppet piston over just a short section of its end side facing the piston collar, and wherein a supporting ring is arranged axially between the high-pressure seal and the first insert in a receiving element, the diameter of which is substantially smaller than the outside diameters of the inserts, the supporting ring likewise being produced from a metallic material having good sliding properties and being produced with a narrow clearance from the end section of the poppet piston and with a large radial clearance from the receiving element.