  We consider the lightlike spontaneous Lorentz invariance violation (SLIV)
appearing through the zero "length-fixing" constraint put on a gauge vector
field, $A_{\mu }A^{\mu }=0$, and discuss its physical consequences in the
framework of a conventional QED and beyond. Again, as in the timelike and
spacelike SLIV cases, $A_{\mu }A^{\mu }=\pm M_{A}^{2}$ ($M_{A}$ is a scale of
SLIV), while this constraint leads to an emergence of the Nambu-Goldstone modes
collected in physical photon, the SLIV itself is still left unobservable unless
gauge invariance in the theory is broken. At the same time, a crucial
difference with the two former cases is that the asymmetrical vacuum
corresponding to the lightlike Lorentz violation appears infinitely degenerated
with all other vacua including the symmetrical one. We show that this
degeneracy can be lifted out by introducing an extra gauge vector field being
sterile with respect to an ordinary matter, though having some potential
couplings with the basic $A_{\mu }$ field. A slight mixing of them makes the
underlying gauge invariance to be partially broken due to which physical
Lorentz invariance occurs broken as well. This may cause a variety of the
Lorentz violating processes some of which are briefly discussed.
