  The crystal structure of a disordered form of Cr$_{1/3}$NbS$_2$ has been
characterized using diffraction and inelastic scattering of synchrotron
radiation. In contrast to the previously reported symmetry (P6$_3$22), the
crystal can be described by a regular twinning of an average P6$_3$ structure
with three disordered positions of the Cr ions. Short-range correlations of the
occupational disorder result in a quite intense and structured diffuse
scattering; a static nature of the disorder was unambiguously attributed by the
inelastic x-ray scattering. The diffuse scattering has been modeled using a
reverse Monte-Carlo algorithm assuming a disorder of the Cr sub-lattice only.
The observed correlated disorder of the Cr sub-lattice reduces the temperature
of the magnetic ordering from 130 K to 88 K and drastically modifies the field
dependence of the magnetization as it is evidenced by the SQUID magnetometery.
We conclude, that in contrast to the helicoidal spin structure assumed for
P6$_3$22 form, the compound under study is ferromagnetically ordered with a
pronounced in-plane anisotropy.
