We obtain error estimates for the numerical approximation of a distributed control problem governed by the stationary Navier - Stokes equations, with pointwise control constraints. We show that the L2- norm of the error for the control is of order h(2) if the control set is not discretized, while it is of order h if it is discretized by piecewise constant functions. These error estimates are obtained for local solutions of the control problem, which are nonsingular in the sense that the linearized Navier - Stokes equations around these solutions de. ne some isomorphisms, and which satisfy a second order su.fficient optimality condition. We establish a second order necessary optimality condition. The gap between the necessary and su.fficient second order optimality conditions is the usual gap known for. finite dimensional optimization problems.