  The Ritus' $E_{p}$ eigenfunction method is extended to the case of spin-1
charged particles in a constant electromagnetic field and used to calculate the
one-loop neutrino self-energy in the strong-field approximation $m_{e}^{2}\ll
eB\ll M_{W}^{2}$. The implications of the obtained results for neutrino
oscillations in the early Universe, assuming the existence of a sufficiently
large ($m_{e}^{2}\ll eB\ll m_{\mu}^{2}$) primordial magnetic field, are
discussed.
