  We compute the full ${\cal O}(\alpha_s^4)$ and ${\cal
O}(\alpha_s^4\log\alpha_s)$ corrections to the potential $V_R(r)$ between the
static color sources, where $V_R(r)$ is defined from the Wilson loop in a
general representation $R$ of a general gauge group $G$. We find a violation of
the Casimir scaling of the potential, for the first time, at ${\cal
O}(\alpha_s^4)$. The effect of the Casimir scaling violation is predicted to
reduce the tangent of $V_R(r)/C_R$ proportionally to specific color factors
dependent on $R$. We study the sizes of the Casimir scaling violation for
various $R$'s in the case $G=SU(3)$. We find that they are well within the
present bounds from lattice calculations, in the distance region where both
perturbative and lattice computations of $V_R(r)$ are valid. We also discuss
how to test the Casimir scaling violating effect.
