  We develop a new method for obtaining branching rules for affine Kac-Moody
Lie algebras at negative integer levels. This method uses fusion rules for
vertex operator algebras of affine type. We prove that an infinite family of
ordinary modules for affine vertex algebra of type A investigated in Adamovi\'c
and O. Per\v{s}e (2008) is closed under fusion. Then we apply these fusion
rules on explicit bosonic realization of level -1 modules for the affine Lie
algebra of type $A_{\ell-1}^{(1)}$, obtain a new proof of complete reducibility
for these representations, and the corresponding decomposition for $\ell \ge
3$. We also obtain the complete reducibility of the associated level -1 modules
for affine Lie algebra of type $C_{\ell}^{(1)}$. Next we notice that the
category of $D_{2 \ell -1}^{(1)}$ modules at level $- 2 \ell +3 $ obtained in
Per\v{s}e (2012) has the isomorphic fusion algebra. This enables us to
decompose certain $E_6 ^{(1)}$ and $F_4 ^{(1)}$--modules at negative levels.
