  We propose a three-dimensional electromagnetic current operator within
light-front dynamics that satisfies a light-front Ward-Takahashi identity for
two-fermion systems. The light-front current operator is obtained by a
quasi-potential reduction of the four-dimensional current operator and acts on
the light-front valence component of bound or scattering states. A relation
between the light-front valence wave function and the four-dimensional
Bethe-Salpeter amplitude both for bound or scattering states is also derived,
such that the matrix elements of the four-dimensional current operator can be
fully recovered from the corresponding light-front ones. The light-front
current operator can be perturbatively calculated through a quasi-potential
expansion, and the divergence of the proposed current satisfies a
Ward-Takahashi identity at any given order of the expansion. In the
quasi-potential expansion the instantaneous terms of the fermion propagator are
accounted for by the effective interaction and two-body currents. We exemplify
our theoretical construction in the Yukawa model in the ladder approximation,
investigating in detail the current operator at the lowest nontrivial order of
the quasi-potential expansion of the Bethe-Salpeter equation. The explicit
realization of the light-front form of the Ward-Takahashi identity is verified.
We also show the relevance of instantaneous terms and of the pair contribution
to the two-body current and the Ward-Takahashi identity.
