  Context. It is generally accepted that, in Classical T Tauri Stars, the
plasma from the circumstellar disc accretes onto the stellar surface with free
fall velocity, and the impact generates a shock. The impact region is expected
to contribute to emission in different spectral bands; many studies have
confirmed that the X-rays arise from the post-shock plasma but, otherwise,
there are no studies in the literature investigating the origin of the observed
UV emission which is apparently correlated to accretion. Aims. We investigated
the effect of radiative heating of the infalling material by the post-shock
plasma at the base of the accretion stream with the aim to identify in which
region a significant part of the UV emission originates. Methods. We developed
a 1D hydrodynamic model describing the impact of an accretion stream onto the
stellar surface; the model takes into account the gravity, the radiative
cooling of an optically thin plasma, the thermal conduction, and the heating
due to absorption of X-ray radiation. The latter term represents the heating of
the infalling plasma due to the absorption of X-rays emitted from the
post-shock region. Results. We found that the radiative heating of the
pre-shock plasma plays a non-negligible role in the accretion phenomenon. In
particular, the dense and cold plasma of the pre-shock accretion column is
gradually heated up to few $10^5$\;K due to irradiation of X-rays arising from
the shocked plasma at the impact region. This heating mechanism does not affect
significantly the dynamics of the post-shock plasma. On the other hand, a
region of radiatively heated gas (that we consider a precursor) forms in the
unshocked accretion column and contributes significantly to UV emission. Our
model naturally reproduces the luminosity of UV emission lines correlated to
accretion and shows that most of the UV emission originates from the precursor.
