The invention relates to an auto-injector for administering a dose of a liquid medicament, comprising: an elongate housing, a syringe with a hollow needle and a bung for displacing the medicament, wherein the syringe is slidably arranged with respect to the housing, spring means for pushing the needle from a covered position inside the housing into an advanced position past the proximal end and for supplying the dose of medicament, activating means for locking and releasing the spring means. The spring means is a torsion spring grounded at one end in the housing and at the other end in a lead nut rotatable about a longitudinal axis but axially constrainable. The lead nut is arranged for translatively moving a piston rod by means of a lead screw thread. The piston rod is prevented from rotating relative to the housing and arranged to be coupled to the bung in order to push it forwards. The lead nut is engaged to the housing in an initial position prior to manual operation of the activating means in a manner to prevent rotation and disengaged from the housing by the activating means upon manual operation.The activating means is a trigger button, arranged at a distal end of the housing. Prior to manual operation the trigger button, the piston rod and the lead nut are coupled for joint translation in proximal direction. In the initial position the lead nut is engaged to the housing by at least one spline feature. The lead nut is arranged to disengage from the housing on translation in proximal direction from the initial position.