  We report the discovery of TOI 263.01 (TIC 120916706), a transiting
substellar object (R = 0.87 RJup) orbiting a faint M3.5~V dwarf (V=18.97) on a
0.56~d orbit. We set out to determine the nature of the TESS planet candidate
TOI 263.01 using ground-based multicolour transit photometry. The host star is
faint, which makes RV confirmation challenging, but the large transit depth
makes the candidate suitable for validation through multicolour photometry. Our
analysis combines three transits observed simultaneously in r', i', and z_s
bands using the MuSCAT2 multicolour imager, three LCOGT-observed transit light
curves in g, r', and i' bands, a TESS light curve from Sector 3, and a
low-resolution spectrum for stellar characterisation observed with the ALFOSC
spectrograph. We model the light curves with PyTransit using a transit model
that includes a physics-based light contamination component that allows us to
estimate the contamination from unresolved sources from the multicolour
photometry. This allows us to derive the true planet-star radius ratio
marginalised over the contamination allowed by the photometry, and, combined
with the stellar radius, gives us a reliable estimate of the object's absolute
radius. The ground-based photometry excludes contamination from unresolved
sources with a significant colour difference to TOI 263. Further, contamination
from sources of same stellar type as the host is constrained to levels where
the true radius ratio posterior has a median of 0.217. The median radius ratio
corresponds to an absolute planet radius of 0.87 RJup, which confirms the
substellar nature of the planet candidate. The object is either a giant planet
or a brown dwarf (BD) located deep inside the so-called "brown dwarf desert".
Both possibilities offer a challenge to current planet/BD formation models and
makes 263.01 an object deserving of in-depth follow-up studies.
