The semileptonic CP asymmetry in B0-B¯0 mixing, asld, is measured in proton-proton collision data, corresponding to an integrated luminosity of 3.0fb-1, recorded by the LHCb experiment. Semileptonic B0 decays are reconstructed in the inclusive final states D-μ+ and D∗-μ+, where the D- meson decays into the K+π-π- final state and the D∗- meson into the D¯0(→K+π-)π- final state. The asymmetry between the numbers of D(∗)-μ+ and D(∗)+μ- decays is measured as a function of the decay time of the B0 mesons. The CP asymmetry is measured to be asld=(-0.02±0.19±0.30)%, where the first uncertainty is statistical and the second systematic. This is the most precise measurement of asld to date and is consistent with the prediction from the standard model.