  The mixing-induced CP-violating phase $\phi_s$ in ${B}^0_s$ and
$\overline{B}^0_s$ decays is measured using the $J/\psi \pi^+\pi^-$ final state
in data, taken from 3\,fb$^{-1}$ 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 $\phi_s=70\pm 68\pm 8$\,mrad. This
result is consistent with the Standard Model expectation and previous
measurements.
