Medicament delivery device comprising: - a front housing and a rear housing connected to each other; - a medicament container disposed within the front housing, wherein the container has a front opening with or for a delivery member for delivering the medicament therethrough and at least one movable stopper; and - a compression spring having a predetermined compressed length and a predetermined outer diameter, wherein the compressed length is the length of the spring when it is in a pre-tensioned state; characterised in that the device further comprises - a first guide means comprising a front part having a predetermined first length, a middle part having a predetermined second length, and a rear part having a predetermined third length, wherein said predetermined third length corresponds to the predetermined compressed length of the compression spring, wherein the front part is arranged to be in contact with the movable stopper, wherein the middle part comprises an annular ledge and first lock-release means, and wherein the rear part is fully surrounded by the compression spring when it is in its pre-tensioned state; and - a second guide means which constitutes a generally tubular inner wall of the rear housing having a predetermined fourth length, a rear end surface, a predetermined inner diameter, and second lock-release means, wherein the predetermined fourth length corresponds to the predetermined first and the predetermined second lengths together, and wherein the predetermined inner diameter of the second guide means is somewhat larger than the predetermined outer diameter of the compression spring, such that the compression spring is fully arranged inside said second guide means between the rear end surface and the annular ledge when it is in its pre-tensioned state. - a front housing and a rear housing connected to each other; - a medicament container disposed within the front housing, wherein the container has a front opening with or for a delivery member for delivering the medicament therethrough and at least one movable stopper; and - a compression spring having a predetermined compressed length and a predetermined outer diameter, wherein the compressed length is the length of the spring when it is in a pre-tensioned state; characterised in that the device further comprises - a first guide means comprising a front part having a predetermined first length, a middle part having a predetermined second length, and a rear part having a predetermined third length, wherein said predetermined third length corresponds to the predetermined compressed length of the compression spring, wherein the front part is arranged to be in contact with the movable stopper, wherein the middle part comprises an annular ledge and first lock-release means, and wherein the rear part is fully surrounded by the compression spring when it is in its pre-tensioned state; and - a second guide means which constitutes a generally tubular inner wall of the rear housing having a predetermined fourth length, a rear end surface, a predetermined inner diameter, and second lock-release means, wherein the predetermined fourth length corresponds to the predetermined first and the predetermined second lengths together, and wherein the predetermined inner diameter of the second guide means is somewhat larger than the predetermined outer diameter of the compression spring, such that the compression spring is fully arranged inside said second guide means between the rear end surface and the annular ledge when it is in its pre-tensioned state.