Here we present three adaptive mesh refinement radiation hydrodynamics simulations that illustrate the impact of momentum transfer from ionising radiation to the absorbing gas on star formation in high-redshift dwarf galaxies. Momentum transfer is calculated by solving the radiative transfer equation with a ray tracing algorithm that is adaptive in spatial and angular coordinates. We find that momentum input partially affects star formation by increasing the turbulent support to a three-dimensional rms velocity equal to the circular velocity of early haloes. Compared to a calculation that neglects radiation pressure, the star formation rate is decreased by a factor of five to 1.8 × 10-2 M⊙ yr-1 in a dwarf galaxy with a dark matter and stellar mass of 2.0 × 108 M ⊙ and 4.5 × 105 M⊙, respectively, when radiation pressure is included. Its mean metallicity of 10-2.1 Z⊙ is consistent with the observed dwarf galaxy luminosity-metallicity relation. In addition to photo-heating in H II regions, radiation pressure further drives dense gas from star forming regions, so supernovae feedback occurs in a warmer and more diffuse medium, launching metal-rich outflows.