The European Union is situated at the border between a federal state and a confederation and it is being considered a federation of states. Flexible integration is related to the appliance of different criteria of integration to the European states and it involves the formation of two large categories of states: the developed states and the less developed states of the Union. The purpose of the present work is to underline the incompatibility between the flexible integration model applied in the European Union and the tendency to transform the Union into a federal state.