An automatic feedback mechanism for a medicament delivery device, which notifies a user of a start of expulsion of medicament from a medicament container, wherein the automatic feedback mechanism comprises: a plunger rod (11; 11-1; 11-2), a tubular extension member having a radial space defined by an inner perimeter and an inner distal surface, wherein the tubular extension member is arranged to receive the plunger rod (11; 11-1; 11-2) in the radial space, and wherein the tubular extension member has flexible tongues configured to releasably lock the plunger rod (11; 11-1; 11-2) in a pre-tensioned state, a tubular operation member arranged to receive the tubular extension member, which tubular operation member is movable relative to the tubular extension member, wherein the tubular operation member in a first position is arranged to press the flexible tongues radially inwards to lock the plunger rod (11; 11-1; 11-2) in the pre-tensioned state, a first resilient member arranged to be received inside the plunger rod (11; 11-1; 11-2) to exert a force in the proximal direction, and a signal generating member (21; 21-1) arranged to partially surround the plunger rod (11; 11-1; 11-2) and having a distal transversal end portion which in a first position is at a distance D from the inner distal surface of the tubular extension member when the plunger rod (11; 11-1; 11-2) is in the pre-tensioned state, wherein in an assembled state, movement of the tubular operation member causes the flexible tongues to expand radially releasing the plunger rod (11; 11-1; 11-2) to travel proximally and allowing the first resilient member to move the signal generating member (21; 21-1) distally the distance D such that the distal transversal end portion hits the inner distal surface generating an audible signal, and wherein the signal generating member (21; 21-1) comprises an elongated U-shaped bracket having two longitudinally extending flexible legs provided with angled radial feet extending radially outward to secure the signal generating member (21; 21-1) in the first position, and characterised in that the plunger rod has an external surface which has a first section provided with a first guide structure arranged to support and flex the signal generating member (21; 21-1) radially outwards, and to guide axial displacement of the signal generating member (21; 21-1), and that in the initial position of the plunger rod (11; 11-1; 11-2) the axial overlap between the first section and the signal generating member (21; 21-1) corresponds to the distance of axial displacement of the plunger rod (11; 11-1; 11-2) necessary for commencement of medicament expulsion. a plunger rod (11; 11-1; 11-2), a tubular extension member having a radial space defined by an inner perimeter and an inner distal surface, wherein the tubular extension member is arranged to receive the plunger rod (11; 11-1; 11-2) in the radial space, and wherein the tubular extension member has flexible tongues configured to releasably lock the plunger rod (11; 11-1; 11-2) in a pre-tensioned state, a tubular operation member arranged to receive the tubular extension member, which tubular operation member is movable relative to the tubular extension member, wherein the tubular operation member in a first position is arranged to press the flexible tongues radially inwards to lock the plunger rod (11; 11-1; 11-2) in the pre-tensioned state, a first resilient member arranged to be received inside the plunger rod (11; 11-1; 11-2) to exert a force in the proximal direction, and a signal generating member (21; 21-1) arranged to partially surround the plunger rod (11; 11-1; 11-2) and having a distal transversal end portion which in a first position is at a distance D from the inner distal surface of the tubular extension member when the plunger rod (11; 11-1; 11-2) is in the pre-tensioned state, wherein in an assembled state, movement of the tubular operation member causes the flexible tongues to expand radially releasing the plunger rod (11; 11-1; 11-2) to travel proximally and allowing the first resilient member to move the signal generating member (21; 21-1) distally the distance D such that the distal transversal end portion hits the inner distal surface generating an audible signal, and wherein the signal generating member (21; 21-1) comprises an elongated U-shaped bracket having two longitudinally extending flexible legs provided with angled radial feet extending radially outward to secure the signal generating member (21; 21-1) in the first position, and characterised in that the plunger rod has an external surface which has a first section provided with a first guide structure arranged to support and flex the signal generating member (21; 21-1) radially outwards, and to guide axial displacement of the signal generating member (21; 21-1), and that in the initial position of the plunger rod (11; 11-1; 11-2) the axial overlap between the first section and the signal generating member (21; 21-1) corresponds to the distance of axial displacement of the plunger rod (11; 11-1; 11-2) necessary for commencement of medicament expulsion.