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 $varLambda^{0}_{b}tovarLambda^{+}_{c}D^{-}_{s}$ 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.
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