  Several Lyman Continuum (LyC) emitters have been detected so far, but their
observed ionising spectra sometimes differ from attenuated stellar spectra
predicted by stellar population synthesis modelling. This discrepancy may be
due to a significant contribution of LyC nebular emission. We aim to quantify
the importance this emission in LyC leakers: its contribution to the ionising
photons budget, and to measurements of LyC escape. To estimate the nebular
contribution to the LyC spectra of galaxies, we run photoionisation models with
Cloudy for a range of BPASS templates, varying the column density of the
surrounding gas, from density-bounded (log(NH$_{\rm{stop}}$/cm$^{-2}$)=16) to
ionisation-bounded (log(NH$_{\rm{stop}}$/cm$^{-2}$)=19) regimes. In the limits
of very optically thin (f$_{\rm{esc}}$ = 1), or thick configurations
(f$_{\rm{esc}}$ = 0), there is no nebular contribution to the emergent LyC
spectra. This contribution matters only at intermediate LyC opacities ($0 <$
f$_{\rm{esc}}$ $< 1$), where it alters the shape of the LyC spectrum
chromatically, so that escape fractions estimates are highly sensitive to the
wavelength range over which they are calculated. We propose a formula to
estimate integrated escape fractions using f$_{\lambda 700}$/f$_{\lambda 1100}$
flux ratios, since this wavelength range is not affected by nebular emission.
Regarding simulations, the boost of hydrogen ionising photons escaping galaxies
is inversely proportional to the stellar escape fractions, but since typical
simulated escape fractions are low, LyC photons escape is important. Nebular
LyC is a non-negligible additional source of ionising photons from galaxies,
which contribution has been overlooked so far in observations and in cosmic
reionisation simulations.
