  The time-integrated $CP$ asymmetry in the decay $D^0 \to K^0_S K^0_S$ is
measured using $3 fb^{-1}$ of proton-proton collision data collected by the
LHCb experiment at centre-of-mass energies of 7 and 8 TeV. The flavour of the
$D^0$ meson is determined by use of the decay $D^{*+} \to D^0 \pi^+$ and its
charge conjugate mode. The result is \[ {\cal A}_{CP} = -0.029 \pm 0.052 \pm
0.022, \] where the first uncertainty is statistical and the second systematic.
The result is consistent with Standard Model expectations and improves the
uncertainty with respect to the only previous measurement of this quantity by
more than a factor of three.
