  Pyroelectric current and magnetoelectric coupling measurements on
polycrystalline samples of the quadruple perovskite
(LaMn$_{3}$)Mn$_{4}$O$_{12}$ give evidence of ferroelectricity driven by the
antiferromagnetic ordering of the $B$-site Mn$^{3+}$ ions at $T_{N,B}$=78 K
with record values of remnant electric polarization up to $P$=0.56 $\mu$C
cm$^{-2}$. X-ray diffraction measurements indicates an anomalous behavior of
the monoclinic $\beta$ angle at $T_{N,B}$, which suggests that $P$ lies in the
$ac$-plane, where the moments are collinear, so we conclude that exchange
striction is the mechanism of spin-driven ferroelectricity. Polarization values
$\sim$3 $\mu$C cm$^{-2}$ are expected in single crystals, which would open the
avenue towards practical multiferroic applications.
