The present invention relates to a dose setting mechanism (201) for a medicament delivery device comprising, a housing (12) having a longitudinal axis； a lead screw (58) positioned with the housing (12)； a locking member (96) rotationally and slidably fixed to the housing (12), the locking member (96) comprising a locking slot (430)； a dose member assembly (66a) comprising a biasing element (420), an inner sleeve (410) and an outer sleeve (400), where the dose member assembly (66a) is arranged coaxially around the locking member (96) and the lead screw (58), the biasing element (420) is operatively engaged with the locking member (96) and the inner sleeve (410), and where the inner sleeve (410) has a radially projecting key (412) configured to travel in the locking slot (430) to engage the lead screw (58) to prevent axial movement of the lead screw (58) when the dose setting mechanism (201) is in a non-activated state.
