The mass function of dwarf satellite galaxies that are observed around Local Group galaxies differs substantially from simulations(1-5) based on cold dark matter: the simulations predict many more dwarf galaxies than are seen. The Local Group, however, may be anomalous in this regard(6,7). A massive dark satellite in an early-type lens galaxy at a redshift of 0.222 was recently found(8) using a method based on gravitational lensing(9,10), suggesting that the mass fraction contained in substructure could be higher than is predicted from simulations. The lack of very low-mass detections, however, prohibited any constraint on their mass function. Here we report the presence of a (1.9 +/- 0.1) x 10(8)M(circle dot) dark satellite galaxy in the Einstein ring system JVAS B1938+666 (ref. 11) at a redshift of 0.881, where M(circle dot) denotes the solar mass. This satellite galaxy has a mass similar to that of the Sagittarius(12) galaxy, which is a satellite of the Milky Way. We determine the logarithmic slope of the mass function for substructure beyond the local Universe to be 1.1(-0.4)(+0.6), with an average mass fraction of 3.3(-1.8)(+3.6) per cent, by combining data on both of these recently discovered galaxies. Our results are consistent with the predictions from cold dark matter simulations(13-15) at the 95 per cent confidence level, and therefore agree with the view that galaxies formed hierarchically in a Universe composed of cold dark matter.