A medicament injection device is disclosed comprising a housing containing a medicament cartridge holder； a rotatable cap assembly； a needle holder holding a needle, the needle holder being releasably supported by the cap assembly, a pre-stressed spring coupled to the medicament cartridge holder； wherein the cap assembly is arranged so that rotation thereof releases the pre-stressed spring, causing the medicament cartridge holder to move axially in a distal direction towards the needle. A method of operating the device is also disclosed.
