  We determine the NNLO electroweak correction to the $e^+ e^-\to
b\bar{b}W^+W^- X$ production cross section near the top-pair production
threshold. The calculation includes non-resonant production of the final state
as well as electroweak effects in resonant top anti-top pair production with
non-relativistic resummation, and elevates the theoretical prediction to NNNLO
QCD plus NNLO electroweak accuracy. We then study the impact of the new
contributions on the top-pair threshold scan at a future lepton collider.
