  Introducing a variable cosmological parameter $\Lambda (t)$ in a geometrical
manner from a 5D Riemann-flat metric, we investigate the origin and evolution
of primordial magnetic fields in the early universe, when the expansion is
governed by a cosmological parameter $\Lambda (t)$ that decreases with time.
Using the gravitoelectromagnetic inflationary formalism, but without the
Feynman gauge, we obtain the power of spectrums for large-scale magnetic fields
and the inflaton field fluctuations during inflation. A very important fact is
that our formalism is {\em naturally non-conformally invariant}.
