This paper considers the interpretation of the substantive test laid down in Article 2 of Regulation 139/2004. It focuses on horizontal mergers in the so-called ‘gap’ cases, which would not result in the creation or the strengthening of a (single or collective) dominant position. In its practice and soft law instruments, the Commission has construed Article 2 in such a way that virtually any transaction involving actual or potential competitors could lead to a finding of a significant impediment to effective competition. Under this approach, the substantive test would be fulfilled, in principle, in every horizontal merger. In CK Telecoms, the General Court crafted an alternative framework that is capable of meaningfully constraining administrative action and ensures that judicial review in EU merger control remains effective.