  We report on an approximately twelve hour long X-ray flare from the low-mass
X-ray binary KS 1731-260. The flare has a rise time of less than 13 min and
declines exponentially with a decay time of 2.7 hours. The flare emission is
well described by black-body radiation with peak temperature of 2.4 keV. The
total energy release from the event is 10^{42} erg (for an assumed distance of
7 kpc). The flare has all the characteristics of thermo-nuclear X-ray bursts
(so-called type I X-ray bursts), except for its very long duration and
therefore large energy release (factor of 1500-4000 longer and 250-425 more
energy than normal type I X-ray bursts from this source). The flare is preceded
by a short and weak X-ray burst, possibly of type I. Days to weeks before the
flare, type I X-ray bursts were seen at a rate of ~3 per day. However, after
the flare type I X-ray bursting ceased for at least a month, suggesting that
the superburst affected the type I bursting behaviour. The persistent emission
is not significantly different during the non-bursting period. We compare the
characteristics of this event with similar long X-ray flares, so-called
superbursts, seen in other sources (4U 1735-44, 4U 1820-30, 4U 1636-53, Ser
X-1, GX 3+1). The event seen from KS 1731-260 is the longest reported so far.
We discuss two possible mechanisms that might cause these superbursts, unstable
carbon burning (as proposed recently) and electron capture by protons with
subsequent capture of the resulting neutrons by heavy nuclei.
