A medicament delivery device comprising: - a tubular housing comprising a distal end and a proximal end; - a medicament container arranged with a medicament delivery member and placed in the housing; - a cap removably arranged on the proximal end of the housing and configured to cover the delivery member; - a drive mechanism configured to act on the medicament container to expel the medicament through the medicament delivery member upon the drive mechanism being triggered; - an activation member arranged coaxially movable in relation to the housing, associated with the drive mechanism and comprising a proximal tubular portion which extends from the proximal end of the housing and wherein for triggering the drive mechanism the activation member is axially moved towards the distal end of the housing after removing the cap from the housing; the proximal tubular portion of the activation member comprising a resilient element arranged with a friction member; and the cap comprising an interaction member configured to radially force the resilient element and thereby cause the friction member to laterally engage with the housing when the cap is arranged on the proximal end of the housing; whereby an accidental axial movement of the activation member is prevented; wherein the proximal tubular portion comprises a lateral cut-out; wherein the cut-out defines the resilient element with a fixed end and a free end which is radially movable in relation to the proximal tubular portion; and wherein the friction member is arranged on the outer surface of the free end of the resilient element; wherein the outer surface of the proximal tubular portion where the resilient element and the friction member are located, is at least partially surrounded by the inner surface of the proximal portion of the housing; wherein when the cap is attached to the housing, the interaction member is configured to provide a pressing force or normal force on the resilient element, so that the friction for retaining the proximal tubular portion is generated. - a tubular housing comprising a distal end and a proximal end; - a medicament container arranged with a medicament delivery member and placed in the housing; - a cap removably arranged on the proximal end of the housing and configured to cover the delivery member; - a drive mechanism configured to act on the medicament container to expel the medicament through the medicament delivery member upon the drive mechanism being triggered; - an activation member arranged coaxially movable in relation to the housing, associated with the drive mechanism and comprising a proximal tubular portion which extends from the proximal end of the housing and wherein for triggering the drive mechanism the activation member is axially moved towards the distal end of the housing after removing the cap from the housing; the proximal tubular portion of the activation member comprising a resilient element arranged with a friction member; and the cap comprising an interaction member configured to radially force the resilient element and thereby cause the friction member to laterally engage with the housing when the cap is arranged on the proximal end of the housing; whereby an accidental axial movement of the activation member is prevented; wherein the proximal tubular portion comprises a lateral cut-out; wherein the cut-out defines the resilient element with a fixed end and a free end which is radially movable in relation to the proximal tubular portion; and wherein the friction member is arranged on the outer surface of the free end of the resilient element; wherein the outer surface of the proximal tubular portion where the resilient element and the friction member are located, is at least partially surrounded by the inner surface of the proximal portion of the housing; wherein when the cap is attached to the housing, the interaction member is configured to provide a pressing force or normal force on the resilient element, so that the friction for retaining the proximal tubular portion is generated.