The heat flux method for measuring laminar burning velocities has been extended to subatmospheric pressures, down to 80 mbar. The new setup is described and adaptations necessary for the new conditions are analyzed. This includes a new burner plate to compensate for the decrease of sensitivity of the measurement at lower pressures, due to a lower density of the gases and hence a smaller heat flux of the flame to the burner. Measurement results of burning velocities of a stoichiometric methane/air mixture with an initial temperature of 295 K in the range 150-1000 mbar are presented including 95% error margins. A comparison with other experimental results and GRI-based numerical calculations is made. Additionally, for a pressure of 200 mbar the burning velocity of methane/air flames has been measured as a function of equivalence ratio. For pressures lower than 150 mbar, it turns out that the effect of unburned mixture, diffusion out of the boundary layer of the flame becomes a concern. An analysis is presented to study the influence of systematical errors arising from this effect.