Supplementary device for a medicament delivery device

A supplementary device configured to be mounted to a medicament delivery device, wherein the supplementary device comprises:

• a casing,
• a button configured to be received by the casing and configured to actuate a trigger of the medicament delivery device, wherein the button is configured to be displaceable from an initial retracted position to an extended position relative to the casing, and
• a button locking member comprises a radial flexible arm that is configured to directly engage with and lock the button in the retracted position of the button to prevent displacement of the button from the retracted position to the extended position when the supplementary device is not mounted to the medicament delivery device,
• wherein the mounting of the supplementary device to the medicament delivery device disengages the flexible arm from the button such that the button is unlocked and released from the retracted position to allow displacement of the button to the extended position.