  Pre-merger gravitational-wave (GW) sky-localisation of binary neutron star
(BNS) and neutron star black hole (NSBH) coalescence events, would enable
telescopes to capture precursors and electromagnetic (EM) emissions around the
time of the merger. We propose a novel astrophysical scenario that could
provide early-warning times of hours to days before coalescence with
sub-arcsecond localisation, provided that these events are gravitationally
lensed. The key idea is that if the BNS/NSBH is lensed, then so must the host
galaxy identified via the EM counterpart. From the angular separation of the
lensed host galaxy images, as well as its redshift and the (foreground) lens
redshift, we demonstrate that we can predict the time delays assuming a
standard lens model. Encouraged by the non-trivial upper limits on the
detection rates of lensed BNS/NSBH mergers that we estimate for upcoming
observing runs of the LIGO-Virgo-Kagra and third generation networks, we assess
the feasibility and benefits of our method. To that end, we study the effect of
limited angular resolution of the telescopes on our ability to predict the time
delays. We find that with an angular resolution of $0.05''$, we can predict
time delays of $> 1$ day with $1\sigma$ error-bar of $\mathcal{O}$(hours) at
best. We also construct realistic time delay distributions of detectable lensed
BNSs/NSBHs to forecast the early-warning times we might expect in the observing
scenarios we consider.
