  We present a comprehensive analysis of the presence of very massive stars
(VMS > $100 M_{\odot}$) in the integrated spectra of 13 UV-bright star-forming
galaxies at $2.2 \lesssim z \lesssim 3.6$ taken with the Gran Telescopio
Canarias (GTC). These galaxies have very high UV absolute magnitudes ($M_{\rm
UV} \simeq -24$), intense star formation (SFR $ \simeq 100-1000$ $M_{\odot}$
yr$^{-1}$), and metallicities in the range of 12+log(O/H) $\simeq8.10-8.50$
inferred from strong rest-optical lines. The GTC rest-UV spectra reveal
spectral features indicative of very young stellar populations with VMS, such
as strong P-Cygni line profiles in the wind lines N~{\sc v} $\lambda 1240$ and
C~{\sc iv} $\lambda 1550$ along with intense and broad He~{\sc ii} $\lambda
1640$ emission with $EW_{0}$ $\simeq 1.40-4.60$ \AA, and FWHM $\simeq
1150-3170$ $km \ s^{-1}$. A Comparison with known VMS-dominated sources and
typical galaxies without VMS reveals that some UV-bright galaxies closely
resemble VMS-dominated clusters (e.g., R136 cluster). The presence of VMS is
further supported by a quantitative comparison of the observed strength of the
He~{\sc ii} emission with population synthesis models with and without VMS,
where models with VMS are clearly preferred. Employing an empirical threshold
for $EW_{0}$ (\heii) $\geq 3.0$ \AA, along with the detection of other
VMS-related spectral profiles (N~{\sc iv} $\lambda 1486, 1719$), we classify
nine out of 13 UV-bright galaxies as VMS-dominated sources. This high incidence
of VMS-dominated sources in the UV-bright galaxy population ($\approx 70\%$)
contrasts significantly with the negligible presence of VMS in typical $L_{\rm
UV}^{*}$ LBGs at similar redshifts ($<1\%$). Our results thus indicate that VMS
are common in UV-bright galaxies, suggesting a different initial mass function
(IMF) with upper mass limits between $175 M_{\odot}$ and $475 M_{\odot}$.
