  The relativistic two body problem is considered in terms of the action
integral in the case of two interacting spinless particles and spin-$1/2$
fermions, interacting by means of vector and scalar fields. The Lagrangians
governing the dynamics of such particles are derived in the momentum
representation. In the Breit frame, when the two-body system is at rest as
whole, the obtained Lagrangians generate the Klein-Gordon-like and Dirac-like
equations in the (3+1) phase space. The derived equations are examined by
studying a para-positronium in a strong magnetic field. It is shown that a
strong magnetic fields result in essential decreasing the para-positronium
life-time.
