  We have explicitly shown that the infrared structure of the full gluon
propagator in QCD is an infite sum over all severe (i.e., more singular than
$1/q^2$) infrared singularities. It reflects the zero momentum modes
enhancement effect in the true QCD vacuum. Its existence exhibits a
characteristic mass (the so-called mass gap), which is responsible for the
scale of nonperturbative dynamics in the QCD ground state. By an infrared
renormalization of a mass gap only, the deep infrared structure of the full
gluon propagator is saturated by the simplest severe infrared singularity, the
famous $(q^2)^{-2}$. So, there is no smooth in the infrared limit the full
gluon propagator. The main dynamical source of severe infrared singularities in
the gluon propagator is the two-loop skeleton term of the corresponding
equation of motion, which contains the four-gluon vertices only. Taking into
account the distribution nature of severe infrared singularities, the gluon
confinement criterion is formulated in a manifestly gauge-invariant way.
