Proof of a conjecture concerning the direct product of bipartite graphs

We prove that if the direct product of two connected bipartite graphs has isomorphic components, then one of the factors admits an automorphism that interchanges its partite sets. This proves a conjecture made by Jha, Klavžar and Zmazek in 1997 [P. Jha, S. Klavzar, B. Zmazek, Isomorphic components of Kronecker product of bipartite graphs, Discussiones Mathematicae Graph Theory 17 (1997) 302–308].