  The filamentation instability (sometimes also referred to as "Weibel") is a
key process in many astrophysical scenario. In the Fireball model for Gamma Ray
Bursts, this instability is believed to mediate collisionless shock formation
from the collision of two plasma shells. It has been known for long that a flow
aligned magnetic field can completely cancel this instability. We show here
that in the general case where there is an angle between the field and the
flow, the filamentation instability can never be stabilized, regardless of the
field strength. The presented model analyzes the stability of two symmetric
counter-streaming cold electron/proton plasma shells. Relativistic effects are
accounted for, and various exact analytical results are derived. This result
guarantees the occurrence of the instability in realistic settings fulfilling
the cold approximation.
