The symmetry breaking of a lean premixed laminar methane/air flame, propagating in an horizontal narrow tube of internal diameter i = 5 mm, is investigated experimentally and numerically. The methane/air flame numerical simulations performed include complex molecular transport and the fully coupled solving of heat transfer at and within the wall. In place of a symmetrical flame, experiments show the stability of a slanted flame with a preferential anchoring at the top. This is observed for both flames freely propagating along isothermal (T = 300 K) walls, and flames stabilized by the incoming flow and thermally coupled with heat transfer inside the walls. Numerical simulations with and without gravity demonstrate its preponderant contribution in the symmetry breaking. The slight flow stratification is sufficient to break the symmetry, with a dominant role of the baroclinic torque, leading to a specific deviation of the incoming streamlines observed both experimentally and numerically.