Lyman- emitters are thought to be young, low-mass galaxies with ages of 108 yr (refs 1, 2). An overdensity of them in one region of the sky (the SSA 22 field) traces out a filamentary structure in the early Universe at a redshift of z 3.1 (equivalent to 15 per cent of the age of the Universe) and is believed to mark a forming protocluster3, 4. Galaxies that are bright at (sub)millimetre wavelengths are undergoing violent episodes of star formation5, 6, 7, 8, and there is evidence that they are preferentially associated with high-redshift radio galaxies9, so the question of whether they are also associated with the most significant large-scale structure growing at high redshift (as outlined by Lyman- emitters) naturally arises. Here we report an imaging survey of 1,100-m emission in the SSA 22 region. We find an enhancement of submillimetre galaxies near the core of the protocluster, and a large-scale correlation between the submillimetre galaxies and the low-mass Lyman- emitters, suggesting synchronous formation of the two very different types of star-forming galaxy within the same structure at high redshift. These results are in general agreement with our understanding of the formation of cosmic structure.