  The one-loop induced top quark decay $t\to u_1\bar{u}_2u_2$ ($u_i=u,c$) is
calculated in the context of the standard model. The dominant contribution to
this top quark decay arises from the Feynman diagrams induced by the off-shell
$tu_1g^*$ vertex, whereas the box diagrams are negligibly small. In contrast
with the on-shell $tu_1g$ vertex, which only gives rise to a pure dipolar
effect, the off-shell $tu_1g^*$ coupling also involves a monopolar term, which
gives a larger contribution than the dipolar one. It is found that the
branching ratio for the three-body decay $t\to u_1\bar{u}_2u_2$ is about of the
same order of magnitude of the two-body decay $t\to u_1 g$, the latter being
slightly larger, which stems from the fact that the three-body decay is
dominated by a monopolar term.
