  It is well-known that General Relativity with positive cosmological constant
can be formulated as a gauge theory with a broken SO(1,4) symmetry. This
symmetry is broken by the presence of an internal space-like vector $V^A$,
$A=0,...,4$, with SO(1,3) as a residual invariance group. Attempts to ascribe
dynamics to the field $V^{A}$ have been made in the literature but so far with
limited success. Regardless of this issue we can take the view that $V^A$ might
actually vary across spacetime and in particular become null or time-like. In
this paper we will study the case where $V^A$ is null. This is shown to
correspond to a Lorentz violating modified theory of gravity. Using the
isomorphism between the de Sitter group and the spatial conformal group,
$SO(1,4)\simeq C(3)$, we show that the resulting gravitational field equations
are invariant under all the symmetries, but spatial translations, of the
conformal group C(3).
