  We calculate the effect that a primordial homogeneous magnetic field, $\B_0$,
will have on the different CMB power spectra due to Faraday rotation.
Concentrating on the $TB$, $EB$ and $BB$ correlations, we forecast the ability
for future CMB polarization experiments to constrain $\B_0$. Our results depend
on how well the foregrounds can be subtracted from the CMB maps, but we find a
predicted error between $\sigma_{\B_0} = 4 \times 10^{-11}$Gauss (for the QUIET
experiment with foregrounds perfectly subtracted) and $3 \times 10^{-10}$Gauss
(with the Clover experiment with no foreground subtraction). These constraints
are two orders of magnitudes better than the present limits on $\B_0$.
