  We present the detection of a $\Delta V\sim$ -10 flare from the ultracool
L2.5 dwarf ULAS J224940.13-011236.9 with the Next Generation Transit Survey
(NGTS). The flare was detected in a targeted search of late-type stars in NGTS
full-frame images and represents one of the largest flares ever observed from
an ultracool dwarf. This flare also extends the detection of white-light flares
to stars with temperatures below 2000 K. We calculate the energy of the flare
to be $3.4^{+0.9}_{-0.7}\times10^{33}$erg, making it an order of magnitude more
energetic than the Carrington event on the Sun. Our data show how the
high-cadence NGTS full-frame images can be used to probe white-light flaring
behaviour in the latest spectral types.
