Case C-376/20 P: Judgment of the Court (Grand Chamber) of 13 July 2023 — European Commission v CK Telecoms UK Investments Ltd, United Kingdom of Great Britain and Northern Ireland, EE 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 — Annulment)