A safety mechanism for a medicament delivery device for preventing activation thereof in the event of mechanical shock, which safety mechanism comprises: a medicament delivery member sleeve, a proximal flange provided at the proximal end of the medicament delivery member sleeve, which proximal flange has a first radially flexible arm extending longitudinally in the distal direction, a housing having a central proximal opening arranged to receive a distal portion of the medicament delivery member sleeve, wherein the medicament delivery member sleeve is axially displaceable inside the housing from an extended position to a retracted position relative to the housing for activating medicament expulsion, and characterized in that it further comprises: a proximally arranged cover having a distal opening forming a channel arranged to receive the medicament delivery member sleeve and the housing, wherein the housing has a proximal portion arranged between the first radially flexible arm and the outer surface of the medicament delivery member sleeve when the medicament delivery member sleeve is displaced from the extended position towards the retracted position, and wherein the channel is dimensioned such that, when the housing is received by the cover, there is a radial distance between the inner surface of the cover and the outer surface of the proximal portion which is less than a radial dimension D of the first radially flexible arm, preventing the medicament delivery member sleeve from obtaining its retracted position, and wherein in the distal direction the radial distance between the inner surface of the cover and the outer surface of the proximal portion decreases from initially being greater than a radial dimension D of a distal end of the first radially flexible arm. a medicament delivery member sleeve, a proximal flange provided at the proximal end of the medicament delivery member sleeve, which proximal flange has a first radially flexible arm extending longitudinally in the distal direction, a housing having a central proximal opening arranged to receive a distal portion of the medicament delivery member sleeve, wherein the medicament delivery member sleeve is axially displaceable inside the housing from an extended position to a retracted position relative to the housing for activating medicament expulsion, and characterized in that it further comprises: a proximally arranged cover having a distal opening forming a channel arranged to receive the medicament delivery member sleeve and the housing, wherein the housing has a proximal portion arranged between the first radially flexible arm and the outer surface of the medicament delivery member sleeve when the medicament delivery member sleeve is displaced from the extended position towards the retracted position, and wherein the channel is dimensioned such that, when the housing is received by the cover, there is a radial distance between the inner surface of the cover and the outer surface of the proximal portion which is less than a radial dimension D of the first radially flexible arm, preventing the medicament delivery member sleeve from obtaining its retracted position, and wherein in the distal direction the radial distance between the inner surface of the cover and the outer surface of the proximal portion decreases from initially being greater than a radial dimension D of a distal end of the first radially flexible arm. a proximally arranged cover having a distal opening forming a channel arranged to receive the medicament delivery member sleeve and the housing, wherein the housing has a proximal portion arranged between the first radially flexible arm and the outer surface of the medicament delivery member sleeve when the medicament delivery member sleeve is displaced from the extended position towards the retracted position, and wherein the channel is dimensioned such that, when the housing is received by the cover, there is a radial distance between the inner surface of the cover and the outer surface of the proximal portion which is less than a radial dimension D of the first radially flexible arm, preventing the medicament delivery member sleeve from obtaining its retracted position, and wherein in the distal direction the radial distance between the inner surface of the cover and the outer surface of the proximal portion decreases from initially being greater than a radial dimension D of a distal end of the first radially flexible arm.