  The low $x$ behaviour of the gluon density $xg(x,\mu^2)$ at scale $\mu^2=2.4$
GeV$^2$ is determined using exclusive $J/\psi$ production data from HERA and
LHCb within the framework of collinear factorisation at next-to-leading order
(NLO). It is shown that in the interval $3\times 10^{-6} < x <10^{-3}$ the
gluon distribution function grows as $xg(x,\mu^2)\propto x^{-\lambda}$ with
$\lambda=0.135\pm 0.006 $. The impact this experimental data will have for the
global parton distribution function (PDF) analyses in this low $x$ domain is
quantified. No indication in favour of parton density saturation is observed.
