  The observed two-point angular correlation function, w(theta), of mJy radio
sources exhibits the puzzling feature of a power-law behaviour up to very large
(almost 10 degrees) angular scales which cannot be accounted for in the
standard hierarchical clustering scenario for any realistic redshift
distribution of such sources. After having discarded the possibility that the
signal can be explained by a high density local source population, we find no
alternatives to assuming that - at variance with all the other extragalactic
populations studied so far, and in particular with optically selected quasars -
radio sources responsible for the large-scale clustering signal were
increasingly less clustered with increasing look-back time, up to at least z=1.
The data are accurately accounted for in terms of a bias function which
decreases with increasing redshift, mirroring the evolution with cosmic time of
the characteristic halo mass, M_{star}, entering the non linear regime. In the
framework of the `concordance cosmology', the effective halo mass controlling
the bias parameter is found to decrease from about 10^{15} M_{sun}/h at z=0 to
the value appropriate for optically selected quasars, 10^{13} M_{sun}/h, at
z=1.5. This suggests that, in the redshift range probed by the data, the
clustering evolution of radio sources is ruled by the growth of large-scale
structure, and that they are associated with the densest environments
virializing at any cosmic epoch. The data provide only loose constraints on
radio source clustering at z>1 so we cannot rule out the possibility that at
these redshifts the clustering evolution of radio sources enters a different
regime, perhaps similar to that found for optically selected quasars. The
dependence of w(theta) on cosmological parameters is also discussed.
