  The CoRoT faint stars channel observed about 163 600 targets to detect
transiting planetary companions. Because CoRoT targets are faint (11< r <16)
and close to the galactic plane, only a small subsample has been observed
spectroscopically. We describe the latest classification scheme used to derive
the spectral type of CoRoT targets, which is based on broadband multi-colour
photometry. We assess the accuracy of this spectral classification for the
first time. We find that the classification method performs better for stars
that were observed during the mission-dedicated photometric ground-based
campaigns.The luminosity class is wrong for less than 7% of the targets.
Generally, the effective temperature of stars classified as early type (O, B,
and A) is overestimated. Conversely, the temperature of stars classified as
later type tends to be underestimated. This is mainly due to the adverse effect
of interstellar reddening. We find that the median error on the effective
temperature is less than 5% for dwarf stars classified with a spectral later
than F0, but it is worse for earlier type stars, with up to 20\% error for A
and late-B dwarfs, and up to 70% for early-B and O-type dwarfs. Similar results
are found for giants, with a median error that is lower than 7% for G- and
later type giants, but greater than 25% for earlier types. Overall, we find an
average median absolute temperature difference |Delta Teff| = 533+\-6 K for the
whole sample of stars classified as dwarfs and |Delta Teff| = 280+\-3 K for the
whole sample of giant stars. The corresponding standard deviation is of about
92+\-5 K for dwarfs and 304+\-4 K for giants. Typically for late-type stars,
this means that the classification is accurate to about half a class.
