  We report here on a fast current-driven instability at relativistic
collisionless shocks, triggered by the perpendicular current carried by the
supra-thermal particles as they gyrate around the background magnetic field in
the shock precursor. We show that this instability grows faster than any other
instability studied so far in this context, and we argue that it is likely to
shape the physics of the shock and of particle acceleration in a broad
parameter range.
