  We evaluate the large scale polarization of the cosmic microwave background
induced, via Thomson scattering prior to decoupling, by nearly scale-invariant
spectra of scalar and tensor metric perturbations, such as those predicted by
most inflationary models of the early Universe. We solve the radiative transfer
equation for the polarized photon distribution function analytically, for
wavelengths of order or larger than the horizon size at decoupling, assuming no
reionization. The induced polarization is proportional to the redshift induced
by the perturbations at decoupling and to the duration of the decoupling
transition. Normalizing the spectra to the quadrupole anisotropy measured by
the COBE satellite, the expected degree of linear polarization is $P\lsim
10^{-7}$, more than two orders of magnitude smaller than current bounds. The
dependence of $P$ with the tilt in the spectrum away from scale invariance is
too weak to be of observational relevance or to provide a sensitive
discrimination between scalar and tensor fluctuations. The variation of $P$
with the angular scale is significant.
