The conditions dictated by a vapour bubble to exist in saturated liquid flowing inside a micro-channel are the main subject of the paper. The liquid flow is laminar and fully established at a constant wall heat flux. Under these conditions, an equilibrium bubble is shown to require a heat flux, which sensitively increases with decreasing channel radius. Given a mass flow density, the hydrodynamic forces acting on the bubble generated in a macro-channel may cause its premature detachment thereby shifting formation of "visible" bubbles towards higher heat fluxes in comparison to macro-channels.