  Future experiments such as SHiP and high-intensity $e^+ e^-$ colliders will
have a superb sensitivity to heavy Majorana neutrinos with masses below $M_Z$.
We show that the measurement of the mixing to electrons and muons of one such
state could imply the discovery of leptonic CP violation in the context of
seesaw models. We quantify in the minimal model the CP discovery potential of
these future experiments, and demonstrate that a 5$\sigma$ CL discovery of
leptonic CP violation would be possible in a very significant fraction of
parameter space.
