  We have made an ab initio three-loop quark model calculation of the
$W$-exchange contribution to the nonleptonic two-body decays of the doubly
charmed baryons $\Xi_{cc}^{++}$ and $\Omega_{cc}^{+}$. The $W$-exchange
contributions appear in addition to the factorizable tree graph contributions
and are not suppressed in general. We make use of the covariant confined quark
model previously developed by us to calculate the tree graph as well as the
$W$-exchange contribution. We calculate helicity amplitudes and quantitatively
compare the tree graph and $W$-exchange contributions. Finally, we compare the
calculated decay widths with those from other theoretical approaches when they
are available.
