  Global linear stability analysis of a self-similar solution describing the
interaction of a relativistic shell with an ambient medium is performed. The
solution is shown to be unstable to convective Rayleigh-Taylor modes having
angular scales smaller than the causality scale. Longer wavelength modes are
stable and decay with time. For modes of sufficiently large spherical harmonic
degree $l$ the dimensionless growth rate scales as $\sqrt{l/\Gamma}$, where
$\Gamma$ is the Lorentz factor of the shell. The instability commences at the
contact interface separating the shocked eject a and shocked ambient gas and
propagates to the shocks. The reverse shock front responds promptly to the in
stability and exhibits rapidly growing distortions at early times. Propagation
to the forward shock is slower, and it is anticipated that the region near the
contact will become fully turbulent before the instability is communicated to
the forward shock. The non-universality of the Blandford-McKee blast wave
solution suggests that turbulence generated by the instability in the shocked
ambient medium may decay slowly with time and may be the origin of magnetic
fields over a long portion of the blast wave evolution. It is also speculated
that the instability may affect the emission from the shocked ejecta in the
early post-prompt phase of GRBs.
