  The black hole mass and accretion rate in Ultraluminous X-ray sources (ULXs)
in external galaxies, whose X-ray luminosities exceed those of the brightest
black holes in our Galaxy by hundreds and thousands of times$^{1,2}$, is an
unsolved problem. Here we report that all ULXs ever spectroscopically observed
have about the same optical spectra apparently of WNL type (late nitrogen
Wolf-Rayet stars) or LBV (luminous blue variables) in their hot state, which
are very scarce stellar objects. We show that the spectra do not originate from
WNL/LBV type donors but from very hot winds from the accretion discs with
nearly normal hydrogen content, which have similar physical conditions as the
stellar winds from these stars. The optical spectra are similar to that of SS
433, the only known supercritical accretor in our Galaxy$^{3}$, although the
ULX spectra indicate a higher wind temperature. Our results suggest that ULXs
with X-ray luminosities of $\sim 10^{40}$ erg s$^{-1}$ must constitute a
homogeneous class of objects, which most likely have supercritical accretion
discs.
