The present disclosure relates to a safety mechanism (27) for a medicament delivery device for preventing activation thereof in the event of mechanical shock. The safety mechanism (27) comprises a medicament delivery member sleeve (7), a proximal flange (9) provided at the proximal end of the medicament delivery member sleeve (7), which proximal flange (9) has a first radially flexible arm (9a) extending longitudinally in the distal direction, a housing (3) having a central proximal opening (3a) arranged to receive a distal portion of the medicament delivery member sleeve (7), wherein the medicament delivery member sleeve (7) is axially displaceable inside the housing (3) from an extended position to a retracted position relative to the housing (3), and a cover (5) having a distal opening (5a) forming a channel arranged to receive the medicament delivery member sleeve (7) and the housing (3). The housing (3) has a proximal portion (3b) arranged between the first radially flexible arm (9a) and the outer surface of the medicament delivery member sleeve (7) when the medicament delivery member sleeve (7) is displaced from the extended position towards the retracted position. The channel is dimensioned such that, when the housing (3) is received by the cover (5), there is a radial distance between the inner surface of the cover (5) and the outer surface of the proximal portion (3b) which is less than a radial dimension of the first radially flexible arm (9a), preventing the medicament delivery member sleeve (7) from obtaining its retracted position.