The use of Unified Modeling Language (UML) analysis/design models on large projects leads to a large number of interdependent UML diagrams. As software systems evolve, those diagrams undergo changes to, for instance, correct errors or address changes in the requirements. Those changes can in turn lead to subsequent changes to other elements in the UML diagrams. Impact analysis is then defined as the process of identifying the potential consequences (side-effects) of a change, and estimating what needs to be modified to accomplish a change. In this article, we propose a UML model-based approach to impact analysis that can be applied before any implementation of the changes, thus allowing an early decision-making and change planning process. We first verify that the UML diagrams are consistent (consistency check). Then changes between two different versions of a UML model are identified according to a change taxonomy, and model elements that are directly or indirectly impacted by those changes (i.e., may undergo changes) are determined using formally defined impact analysis rules (written with Object Constraint Language). A measure of distance between a changed element and potentially impacted elements is also proposed to prioritize the results of impact analysis according to their likelihood of occurrence. We also present a prototype tool that provides automated support for our impact analysis strategy, that we then apply on a case study to validate both the implementation and methodology.