This paper proposes an approach to prove reachability properties of the form AG psi => EF phi using substitution refinement in classical B. Such properties denote that there exists an execution path for each state satisfying psi to a state satisfying phi. These properties frequently occur in security policies and information systems. We show how to use Morgan's specification statement to represent a property and refinement laws to prove it. The idea is to construct by stepwise refinement a program whose elementary statements are operation calls. Thus, the execution of such a program provides an execution satisfying AG psi => EF phi. Proof obligations are represented using assertions (ASSERT clause of B) and can be discharged using Atelier B