ONE of the most direct methods of constraining the epoch at which the first galaxies formed-and thereby to constrain the age of the Universe-is to identify and date the oldest galaxies at high redshift, But most distant galaxies have been identified on the basis of their abnormal brightness in some spectral region(1-4); such selection criteria are biased towards objects with pronounced nuclear activity or young star-forming systems, in which the spectral signature of older stellar populations will be concealed, Here we report the discovery of a weak and extremely red radio galaxy (53W091) at z = 1.55, and present spectroscopic evidence that its red colour results from a population of old stars, Comparing our spectral data with models of the evolution of stellar populations, we estimate that we are observing this galaxy at least 3.5 Gyr after star-formation activity ceased, This implies an extremely high formation redshift (z > 4) for 53W091 and, by inference, other elliptical galaxies. Moreover, the age of 53W091 is greater than the predicted age of the Universe at z = 1.55, under the assumption of a standard Einstein-de Sitter cosmology (for any Hubble constant greater than 50 km s(-1) Mpc(-1)), indicating that this cosmological model can be formally excluded.