Elliptical galaxies are thought to have formed most of their stars in a rapid burst in the early Universe(1), but an unambiguous example of a ’primaeval’ elliptical galaxy (one undergoing its first major burst of star formation) has yet to be discovered. High-redshift radio galaxies are among the most promising candidates(2,3), because their low-redshift counterparts are identified exclusively with ellipticals, but the presence of an active nucleus complicates the analysis of their evolutionary state from optical-infrared observations(3-5). The failure of optical searches to detect primaeval ellipticals(6-9) suggests that they may be very dusty, prompting us to search for thermal emission from the dust, which will be redshifted to submillimetre wavelengths in our reference frame. Our detection of submillimetre emission from the radio galaxy 4C41.17, reported here, suggests that it contains a large mass of dust, probably located in a dust lane obscuring the centre of the galaxy(10-14). The observations are consistent with the recent occurrence of a massive burst of star formation, but probably not the first such episode. We conclude that this galaxy was already in the final stages of its formation at a look-back time of 12-15 billion years.