  We show that in the presence of large scale primordial hypermagnetic fields,
it is possible to generate a large amount of CP violation to explain the baryon
to entropy ratio during the electroweak phase transition within the standard
model. The mechanism responsible for the existence of a CP violating asymmetry
is the chiral nature of the fermion coupling to the background field in the
symmetric phase which can be used to construct two-fermion interference
processes in analogy to the Bohm-Aharanov effect. We estimate that for strong
hypermagnetic fields B_Y=(0.3-0.5)T^2 the baryon to entropy ratio can be
rho_B/s=(3 - 6)X10^-11 for slowly expanding bubble walls.
