Submillimetre-bright galaxies at high redshift are the most luminous, heavily star-forming galaxies in the Universe1 and are characterized by prodigious emission in the far-infrared, with a flux of at least five millijanskys at a wavelength of 850micrometres. Theyreside inhaloes with masses about 10(13) times that of the Sun2, have low gas fractions compared to main-sequence disks at a comparable redshift3, trace complex environments(4,5) and are not easily observable at optical wavelengths(6). Their physical origin remains unclear. Simulations have been able to form galaxies with the requisite luminosities, but have otherwise been unable to simultaneously match the stellar masses, star formation rates, gas fractions and environments(7-10). Here we report a cosmological hydrodynamic galaxy formation simulation that is able to forma submillimetre galaxy that simultaneously satisfies the broad range of observed physical constraints. We find that groups of galaxies residing in massive dark matter haloes have increasing rates of star formation that peak at collective rates of about 500-1,000 solarmasses per year at redshifts of two to three, by which time the interstellar medium is sufficiently enriched with metals that the region may be observed as a submillimetre-selected system. The intense star formation rates are fuelled in part by the infall of a reservoir gas supply enabled by stellar feedback at earlier times, not through major mergers. With a lifetime of nearly a billion years, our simulations show that the submillimetre-bright phase of high-redshift galaxies is prolonged and associated with significant mass buildup in early-Universe proto-clusters, and that many submillimetre- bright galaxies are composed of numerous unresolved components (for which there is some observational evidence(11)).