  We report the first observation of the decay $\Lambda^{+}_{c} \rightarrow p
K^{+} \pi^{-}$ using a 980 $\mathrm{fb^{-1}}$ data sample collected by the
Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider. This is the
first doubly Cabibbo-suppressed decay of a charmed baryon to be observed. We
measure the branching ratio of this decay with respect to its Cabibbo-favored
counterpart to be $\mathcal{B}(\Lambda^{+}_{c} \rightarrow p K^{+}
\pi^{-})/\mathcal{B}(\Lambda^{+}_{c} \rightarrow p K^{-}
\pi^{+})=(2.35\pm0.27\pm0.21)\times10^{-3}$, where the uncertainties are
statistical and systematic, respectively.
