a . Merger control i. Introduction Based on a system of prior notification , Regulation 139/2004 is rather intended to prevent future concentrations from significantly impeding effective competition in the EU. This has important procedural consequences which in many respects distinguish control of concentrations from infringement proceedings under Regulation 1/2003. ii. Notification of concentrations ‘having a Community dimensio n’ The compulsory one-stop shop monitoring system which Regulation 139/2004 sets up concerns exclusively concentrations having a Community dimension, that is to say concentrations entailing significant structural changes, the impact of which on the market goes beyond the national borders of any one Member State. This is because EU powers can be exercised only within the limits of the principle of ...