  The history of reionisation is highly dependent on the ionising properties of
high-redshift galaxies. It is therefore important to have a solid understanding
of how the ionising properties of galaxies are linked to physical and
observable quantities. In this paper, we use the First Light and Reionisation
Epoch Simulations (FLARES) to study the Lyman-continuum (LyC, i.e.
hydrogen-ionising) emission of massive ($M_*>10^8\,\mathrm{M_\odot}$) galaxies
at redshifts $z=5-10$. We find that the specific ionising emissivity (i.e.
intrinsic ionising emissivity per unit stellar mass) decreases as stellar mass
increases, due to the combined effects of increasing age and metallicity.
FLARES predicts a median ionising photon production efficiency (i.e. intrinsic
ionising emissivity per unit intrinsic far-UV luminosity) of
$\log_{10}(\xi_{\rm ion}\rm{/erg^{-1}Hz})=25.40^{+0.16}_{-0.17}$, with values
spanning the range $\log_{10}(\xi_{\rm ion}\rm{/erg^{-1}Hz})=25-25.75$. This is
within the range of many observational estimates, but below some of the
extremes observed. We compare the production efficiency with observable
properties, and find a weak negative correlation with the UV-continuum slope,
and a positive correlation with the OIII equivalent width. We also consider the
dust-attenuated production efficiency (i.e. intrinsic ionising emissivity per
unit dust-attenuated far-UV luminosity), and find a median of
$\log_{10}(\xi_{\rm ion}\rm{/erg^{-1}Hz})\sim25.5$. Within our sample of
$M_*>10^8\,\mathrm{M_\odot}$ galaxies, it is the stellar populations in low
mass galaxies that contribute the most to the total ionising emissivity. Active
galactic nuclei (AGN) emission accounts for $10-20$ % of the total emissivity
at a given redshift, and extends the LyC luminosity function by $\sim0.5$ dex.
