  We study the statistics and cosmic evolution of massive black hole seeds
formed during major mergers of gas-rich late-type galaxies. Generalizing the
results of the hydro-simulations from Mayer et al. 2010, we envision a scenario
in which a supermassive star can form at the center of galaxies that just
experienced a major merger owing to a multi-scale powerful gas inflow, provided
that such galaxies live in haloes with masses above 10^{11} Msun, are gas-rich
and disc-dominated, and do not already host a massive black hole. We assume
that the ultimate collapse of the supermassive star leads to the rapid
formation of a black hole of 10^5 Msun following a quasi-star stage. Using a
model for galaxy formation applied to the outputs of the Millennium Simulation,
we show that the conditions required for this massive black hole formation
route to take place in the concordance LambdaCDM model are actually common at
high redshift, and can be realized even at low redshift. Most major mergers
above z~4 in haloes with mass > 10^{11} Msun can lead to the formation of a
massive seed and, at z~2, the fraction of favourable mergers decreases to about
half. Interestingly, we find that even in the local universe a fraction (~20%)
of major mergers in massive haloes still satisfy the conditions for our massive
black hole formation route. Those late events take place in galaxies with a
markedly low clustering amplitude, that have lived in isolation for most of
their life, and that are experiencing a major merger for the first time. We
predict that massive black hole seeds from galaxy mergers can dominate the
massive end of the mass function at high (z>4) and intermediate (z~2) redshifts
relative to lighter seeds formed at higher redshift, for example, by the
collapse of Pop III stars. Finally, a fraction of these massive seeds could
lie, soon after formation, above the MBH-MBulge relation.
