The mixing-induced CP-violating phase φ[subscript s] in B[superscript 0][subscript s] and [bar over B][superscript 0][subscript s] decays is measured using the J/ψ π[superscript +]π[superscript -] final state in data, taken from 3 fb[superscript -1] of integrated luminosity, collected with the LHCb detector in 7 and 8 TeV centre-of-mass pp collisions at the LHC. A time-dependent flavour-tagged amplitude analysis, allowing for direct CP violation, yields a value for the phase φ[subscript s] = 70 ± 68 ± 8 mrad. This result is consistent with the Standard Model expectation and previous measurements. © 2014 The Authors.National Science Foundation (U.S.)