  Top quark decays are of particular interest as a mean to test the standard
model (SM) predictions, both for the dominant ($t\to b+W$) and rare decays
($t\to q+W, cV, cVV,c\phi^0,bWZ$). As the latter are highly suppressed, they
become an excellent window to probe the predictions of theories beyond the SM.
In particular, in this paper, we evaluate the corrections from new physics to
the CKM-suppressed SM top quark decay $t\to q+W$ ($q=d,s$), both within the
effective lagrangian approach and the MSSM and we discuss the perspectives to
probe those predictions at the ILC.
