  We present observations with XMM-Newton of M33 X-8, the ultraluminous X-ray
source (L_{0.5-10 keV} ~ 2x10^39 erg/s) closest to the centre of the galaxy.
The best-fit model is similar to the typical model of Galactic black holes in
very high state. Comparison with previous observations indicates that the
source is still in a very high state after about 20 years of observations. No
state transition has been observed even during the present set of XMM-Newton
observations. We estimate the lower limit of the mass of the black hole >6
M_sun, but with proper parameters taking into account different effects, the
best estimate becomes 12 M_sun. Our analysis favours the hypothesis that M33
X-8 is a stellar mass black hole candidate, in agreement with the findings of
other authors. In addition, we propose a different model where the high
luminosity of the source is likely to be due to orientation effects of the
accretion disc and anisotropies in the Comptonized emission.
