  The $\varLambda^0_b \to \varLambda^+_c K^+ K^- \pi^-$ decay is observed for
the first time using a data sample of proton-proton collisions at
centre-of-mass energies of $\sqrt{s}=7$ and 8 $\rm{TeV}$ collected by the
$\mbox{LHCb}$ detector, corresponding to an integrated luminosity of
$3{\rm{fb}^{-1}}$. The ratio of branching fractions between the $\varLambda^0_b
\to \varLambda^+_c K^+ K^- \pi^-$ and the
$\varLambda^{0}_{b}\to\varLambda^{+}_{c}D^{-}_{s}$ decays is measured to be
\begin{equation*} \frac{\mathcal{B} ( \varLambda^0_b \to \varLambda^+_c K^+ K^-
\pi^-) } {\mathcal{B} ( \varLambda^0_b \to \varLambda^+_c D^-_s)} = (9.26 \pm
0.29 \pm 0.46 \pm 0.26)\times10^{-2}, \end{equation*} where the first
uncertainty is statistical, the second systematic and the third is due to the
knowledge of the $D^-_s \to K^+ K^- \pi^-$ branching fraction. No structure on
the invariant mass distribution of the $\varLambda^+_c K^+$ system is found,
consistent with no open-charm pentaquark signature.
