  The decay $B_c\rightarrow J/\psi K^+ K^- \pi^+$ is observed for the first
time, using proton-proton collisions collected with the LHCb detector
corresponding to an integrated luminosity of 3fb$^{-1}$. A signal yield of
$78\pm14$ decays is reported with a significance of 6.2 standard deviations.
The ratio of the branching fraction of $\B_c \rightarrow J/\psi K^+ K^- \pi^+$
decays to that of $B_c \rightarrow J/\psi \pi^+$ decays is measured to be
$0.53\pm 0.10\pm0.05$, where the first uncertainty is statistical and the
second is systematic.
