A medicament delivery device having a proximal end and a distal end, said medicament delivery device comprising: - a front housing and a rear housing interconnected and axially displaceable relative to each other between an extended position and a retracted position; - a plunger rod arranged to act on a plunger of a medicament container; - a drive mechanism arranged to drive said plunger rod; - an actuator connected to the front and rear housings (8, 1) respectively and axially movable in said front and rear housings (8, 1) between a proximal end position and a distal end position, wherein the actuator is biased by a first biasing element towards said distal end position; - a drive sleeve releasably connected to said actuator and arranged to act on said drive mechanism for driving said plunger rod; characterised in that - a locking ring is rotatably arranged at a distal end of the rear housing and in threaded connection with an outer surface of the actuator, wherein the locking ring is prevented from axial movement relative to the rear housing; - a release button is arranged on the rear housing movable between an active and an inactive state, wherein the release button in said active state allows unidirectional rotational movement of the locking ring relative to the rear housing such that the actuator can move axially towards said proximal end position under rotational movement of the locking ring and wherein the release button in said inactive state allows bidirectional rotational movement of the locking ring relative to the rear housing such that the actuator can move axially towards said proximal end position or towards said distal end position under rotational movement of the locking ring. - a front housing and a rear housing interconnected and axially displaceable relative to each other between an extended position and a retracted position; - a plunger rod arranged to act on a plunger of a medicament container; - a drive mechanism arranged to drive said plunger rod; - an actuator connected to the front and rear housings (8, 1) respectively and axially movable in said front and rear housings (8, 1) between a proximal end position and a distal end position, wherein the actuator is biased by a first biasing element towards said distal end position; - a drive sleeve releasably connected to said actuator and arranged to act on said drive mechanism for driving said plunger rod; - a locking ring is rotatably arranged at a distal end of the rear housing and in threaded connection with an outer surface of the actuator, wherein the locking ring is prevented from axial movement relative to the rear housing; - a release button is arranged on the rear housing movable between an active and an inactive state, wherein the release button in said active state allows unidirectional rotational movement of the locking ring relative to the rear housing such that the actuator can move axially towards said proximal end position under rotational movement of the locking ring and wherein the release button in said inactive state allows bidirectional rotational movement of the locking ring relative to the rear housing such that the actuator can move axially towards said proximal end position or towards said distal end position under rotational movement of the locking ring.