This work introduces a stabilised finite element formulation for the Stokes
flow problem with a nonlinear slip boundary condition of friction type. The
boundary condition is enforced with the help of an additional Lagrange
multiplier and the stabilised formulation is based on simultaneously
stabilising both the pressure and the Lagrange multiplier. We establish the
stability and the a priori error analyses, and perform a numerical convergence
study in order to verify the theory