  We calculate the dynamical relativistic corrections, originating from
radiative one-gluon-exchange, to the leptonic decay width of heavy quarkonia in
the framework of a covariant formulation of Light-Front Dynamics. Comparison
with the non-relativistic calculations of the leptonic decay width of J=1
charmonium and bottomonium S-ground states shows that relativistic corrections
are large. Most importantly, the calculation of these dynamical relativistic
corrections legitimate a perturbative expansion in $\alpha_s$, even in the
charmonium sector. This is in contrast with the ongoing belief based on
calculations in the non-relativistic limit. Consequences for the ability of
several phenomenological potential to describe these decays are drawn.
