We report the first absolute measurement of the branching fraction of $Lambda^+_{c}rightarrow Lambda e^+ u_e$. This measurement is based on 567 pb$^{-1}$ of $e^+e^-$ annihilation data produced at $sqrt{s}=4.599$ GeV, which is just above the $Lambda^+_cbar{Lambda}^-_c$ threshold. The data were collected with the BESIII detector at the BEPCII storage rings. The branching fraction is determined to be $mathcal B({Lambda^+_crightarrow Lambda e^+ u_e})=(3.63pm0.38({rm stat})pm0.20({rm syst}))%$, representing a more than twofold improvement in precision upon previously published results. As the branching fraction for $Lambda^+_{c}rightarrow Lambda e^+ u_e$ is the benchmark for those of other $Lambda^+_c$ semileptonic channels, our result provides a unique test of different theoretical models, which is the most stringent to date.
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