In role-based access control models, delegation of authorityinvolves delegating roles that a user can assume or the set of permissions that he can acquire, to other users. Several role-based delegation models have been proposed in the literature. However, these models consider only delegation in presence of the role type, which have some inherent limitations to task delegation in workflow systems. In this paper, we address task delegation in a workflow and elaborate a security model supporting delegation constraints. Delegation constraints express security requirements with regards to task's resources, user's assignment and privileges (delegation of authority). Further, we show how, using a role-based security model, we inject formalised delegation constraints to compute delegation principals with their respective privileges.