A search for the lepton flavour violating decay + is performed with the LHCb experiment. The data sample corresponds to an integrated luminosity of 1.0 fb1 of proton-proton collisions at a centre-of-mass energy of 7 TeV and 2.0 fb1 at 8 TeV. No evidence is found for a signal, and a limit is set at 90% confidence level on the branching fraction, +<4.6×108$$ \mathrm{\mathcal{B}}\left({\tau}^{-}\to {\mu}^{-}{\mu}^{+}{\mu}^{-}\right)<4.6\times {10}^{-8} $$.
