  When the ions are accelerated by the radiation pressure of the laser pulse,
their velocity can not exceed the laser group velocity, in the case when it is
less than the speed of light in vacuum. This is demonstrated in two cases
corresponding to the thin foil target irradiated by a high intensity laser
light and to the hole boring by the laser pulse in the extended plasma
accompanied by the collisionless shock wave formation. It is found that the
beams of accelerated at the collisionless shock wave front ions are unstable
against the Buneman-lke and the Weibel-like instabilities which result in the
ion energy spectrum broadening.
