  Collimated relativistic outflows, or jets, are amongst the most energetic and
relativistic phenomena in the Universe. They are associated with supermassive
black holes in distant active galactic nuclei (AGN), accreting black holes and
neutron stars in binary systems and are believed to be responsible for
gamma-ray bursts. The physics of these jets,however, remains something of a
mystery in that their bulk velocities, compositions and energetics remain
poorly determined. Here we report the discovery of an ultra-relativistic
outflow from a binary accreting neutron star accreting gas within a binary
system. The velocity of the outflow is comparable to the fastest-moving flows
observed from active galactic nuclei, and its strength is modulated by the rate
of accretion of material onto the neutron star. Shocks are energized further
downstream in the flow, which are themselves moving with mildly relativistic
bulk velocities and are the sites of the observed synchrotron emission from the
jet. We conclude that the generation of such highly relativistic outflows does
not require properties unique to black holes, such as an event horizon.
