Judgment of the Court (Grand Chamber) of 13 July 2023.#European Commission v CK Telecoms UK Investments Ltd.#Appeal – Competition – Regulation (EC) No 139/2004 – Control of concentrations of undertakings – Mobile telecommunications services – Decision declaring a concentration incompatible with the internal market – Oligopolistic market – Significant impediment to effective competition – Non-coordinated effects – Standard of proof – European Commission’s margin of discretion with regard to economic matters – Limits of judicial review – Guidelines on horizontal mergers – Factors relevant to demonstrating a significant impediment to effective competition – Concepts of ‘important competitive force’ and ‘close competitors’ – Closeness of competition between the parties to the concentration – Quantitative analysis of the effects of the proposed concentration on prices – Efficiencies – Distortion – Complaint raised by the General Court of the European Union of its own motion