[en] In this paper we contrasted two cosmological perturbation theory formalisms, the 1 + 3 covariant gauge invariant and the gauge invariant, by comparing the gauge invariant variables associated with the magnetic field defined in each approach. In the first part we give an introduction to each formalism assuming the presence of a magnetic field. We found that gauge invariant quantities defined by the 1 + 3 covariant approach are related to spatial variations of the magnetic field (defined in the gauge invariant formalism) between two closed fundamental observers. This relation was computed by choosing the comoving gauge in the gauge invariant approach in a magnetized universe. Furthermore, we have derived the gauge transformations for electromagnetic potentials in the gauge invariant approach, and the Maxwell equations have been written in terms of these potentials. (paper)