  Gamma Ray Bursts (GRBs) are characterized by ultra-relativistic outflows,
while supernovae are generally characterized by non-relativistic ejecta. GRB
afterglows decelerate rapidly usually within days, because their low-mass
ejecta rapidly sweep up a comparatively larger mass of circumstellar material.
However supernovae, with heavy ejecta, can be in nearly free expansion for
centuries. Supernovae were thought to have non-relativistic outflows except for
few relativistic ones accompanied by GRBs. This clear division was blurred by
SN 2009bb, the first supernova with a relativistic outflow without an observed
GRB. Yet the ejecta from SN 2009bb was baryon loaded, and in nearly-free
expansion for a year, unlike GRBs. We report the first supernova discovered
without a GRB, but with rapidly decelerating mildly relativistic ejecta, SN
2012ap. We discovered a bright and rapidly evolving radio counterpart driven by
the circumstellar interaction of the relativistic ejecta. However, we did not
find any coincident GRB with an isotropic fluence of more than a sixth of the
fluence from GRB 980425. This shows for the first time that central engines in
type Ic supernovae, even without an observed GRB, can produce both relativistic
and rapidly decelerating outflows like GRBs.
