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The most massive galaxies and the richest clusters are believed to have emerged from regions with the largest enhancements of mass density relative to the surrounding space. Distant radio galaxies may pinpoint the locations of the ancestors of rich clusters, because they are massive systems associated with ‘overdensities’ of galaxies that are bright in the Lyman-a line of hydrogen. A powerful technique for detecting high-redshift galaxies is to search for the characteristic ‘Lyman break’ feature in the galaxy colour, at wavelengths just shortwards of Lya, which is due to absorption of radiation from the galaxy by the intervening intergalactic medium. Here we report multicolour imaging of the most distant candidate protocluster, TN J1338-1942 at a redshift z ˜ 4.1. We find a large number of objects with the characteristic colours of galaxies at that redshift, and we show that this excess is concentrated around the targeted dominant radio galaxy. Our data therefore indicate that TN J1338-1942 is indeed the most distant cluster progenitor of a rich local cluster, and that galaxy clusters began forming when the Universe was only ten per cent of its present age. [ABSTRACT FROM AUTHOR]