There is disclosed a valve assembly for a lung demand regulator of a breathing apparatus. The valve assembly comprises a regulator valve apparatus for regulating a flow of breathing gas, the regulator valve apparatus being actuatable between a closed position in which no flow is permitted and an open position in which flow is permitted. The valve assembly further comprises a lockout mechanism configurable in: i) a lockout configuration in which the regulator valve apparatus is secured in the closed position, and ii) a release configuration in which the regulator valve apparatus is freely moveable. The lockout mechanism comprises an axially compressible spring element. The spring element is configured to be laterally deflected in a first direction in the lockout configuration and laterally deflected in a second direction different from the first direction in the release configuration. The lockout mechanism is configured such that the spring element is axially compressed during movement of the lockout mechanism between the release configuration and the lockout configuration. Also disclosed is a lung demand regulator and a breathing apparatus comprising a lung demand regulator.