Here we present an ab initio study of surface states exposed to an external electric field. We demonstrate that the external field affects both the vacuum decay rates and the dispersion relations of surface states. In particular, the Cu(111) surface state band bottom is shifted by means of the external electric field. Spin-selective screening of spin-polarized surface states on a Co bilayer on Cu(111) leads to the possibility of reversing the sign of the surface-state spin polarization.