  We show the unique existence of solutions to stationary Navier-Stokes
equations with small singular external forces belonging to a critical space. To
the best of our knowledge, this is the largest critical space that is available
up to now for this kind of existence. This result can be viewed as the
stationary counterpart of the existence result obtained by H. Koch and D.
Tataru for the free non-stationary Navier-Stokes equations with initial data in
BMO$^{-1}$. The stability of the stationary solutions in such spaces is also
obtained by a series of sharp estimates for resolvents of a singularly
perturbed operator and the corresponding semigroup.
