A search for the lepton flavour violating decay τ → μμ+μ is performed with the LHCb experiment. The data sample corresponds to an integrated luminosity of 1.0fb1 of proton-proton collisions at a centre-of-mass energy of 7TeV and 2.0fb1 at 8 TeV. No evidence is found for a signal, and a limit is set at 90% confidence level on the branching fraction, B(τ → μμ+μ) < 4.6 × 108.