The present invention relates to downhole injection tool (1) for injecting an injection fluid into an annular space (5a) surrounding the downhole injection tool and enclosed by an inside wall (3a) of a borehole or a well tubular structure (3). The downhole injection tool comprises an injection unit (1a) comprising a first expandable cup (118a) adapted to provide a first seal (119a) against the inside wall, a second expandable cup (118b) adapted to provide a second seal (119b) against the inside wall, the two cups, in an expanded state, together defining an isolated zone (5b) of the annular space, at least one pipe element (111) extending in a longitudinal direction (13) between the two cups, the pipe element providing a fluid passage (108) between an inlet (104) arranged in one end of the pipe element and an outlet (125) arranged in the pipe element in between the cups, the second expandable cup being slidably connected with the pipe element and displaced in the longitudinal direction away from the first expandable cup under the influence of the injection fluid injected into the isolated zone, whereby a distance d between the two cups is increased, wherein the injection unit further comprises a retainer sleeve (112) being slidably arranged around the expandable cups to prevent unintentional expansion of the expandable cups during insertion of the downhole injection tool, the retainer sleeve is slidable in the longitudinal direction, and the expandable cups are released by movement of the retainer sleeve in the longitudinal direction. Furthermore, the present invention relates to a downhole system comprising the downhole injection tool as well as to a method for casting a cement plug downhole.