Leonidas Theodosiou advises on competition law and litigation. He is highly recommended by leading directories and is uniquely recognized as a leading Next Generation Partner by The Legal 500 UK in both “EU and Competition” and “Competition Litigation” categories. Global Competition Review identified Leo as one of the world’s Top 40 Lawyers Under 40 in foreign direct investment (FDI) regulation. He received Lexology’s 2024 Client’s Choice Award for best UK competition lawyer, noted for “add[ing] real value to clients’ businesses above and beyond the other players in the market.” Leo is one of few lawyers globally who are fully qualified to practice law in the United Kingdom, European Union, and United States.
Leo is a partner on the firm’s “Highly Commended” competition team in London (Legal Business Awards, 2024). He advises on all aspects of UK, EU, and international competition law and competition litigation. Additionally, he counsels clients on FDI matters and the EU regulation of state aid and foreign subsidies.
Leo is praised by clients for his “ability to explain complex legal matters in a way that can be easily understood by non-lawyers,” and for being “extremely skilled and responsive, and [for] bring[ing] a real depth of experience in complex and challenging areas.” It has been noted that he “always goes the extra mile for clients. He really gets to understand the business requirements and commercial context; looking for solutions but also not afraid to tell you the hard truth when you need to hear it.” In relation to merger control, clients say he offers “excellent support and is always on hand during a transaction to offer top class insights which allow for clear and fully informed decision making.”
In 2021, Leo was featured in the Festival of Talent edition of The Lawyer for his contribution in securing Sainsbury’s victory in the UK Supreme Court in its interchange fee litigation against Visa. He is a member of the Morgan Lewis team that was awarded Behavioural Matter of the Year—Europe, Challenges to CMA Search Warrants, by Global Competition Review in 2018.
Prior to joining Morgan Lewis, Leo spent several years practicing with a Magic Circle law firm, resident in Brussels. In 2018, Leo was seconded to Tata Steel—one of the world’s largest steelmakers—where he served as the organization’s in-house counsel dedicated to antitrust.
Leo holds postgraduate degrees in law from Harvard Law School and the University of Cambridge (Trinity Hall). He has received significant awards and fellowships in recognition of his research and academic achievements, including a Fulbright Fellowship by the US Department of State, and was selected to deliver the 2013 Commencement Speech at Harvard Law School.
Leo is the vice chair of the American Bar Association’s Privacy and Information Security Committee. He frequently speaks and writes about data protection issues, in particular on the interplay between antitrust and data protection.
Leo co-leads the firm’s First Generation Lawyer Network, which seeks to provide “first generation” lawyers with the tools necessary to be successful at all stages of their careers.
Merger & FDI Control
Competition Litigation
Antitrust Investigations
State Aid Control
Next Generation Partner, Corporate and Commercial: EU and Competition, The Legal 500 UK (2024, 2025)
Next Generation Partner, Disputes: Competition Litigation, The Legal 500 UK (2025)
Winner, Client’s Choice Award for best competition lawyer in England, Lexology (2023)
Recognized, 40 Under 40, Global Competition Review FIC (2023)
Highly Commended, Competition Team of the Year, Legal Business Awards (2024)
Highly Commended, Competition Team of the Year, The Lawyer Awards (2021, 2022)
Shortlisted, Financial Times Innovative Lawyers Awards Europe (2023, 2024)
Vice Chair, ABA Committee on Privacy & Information Security
Member, Behavioural Matter of the Year—Europe, Challenges to CMA Search Warrants, Global Competition Review (2018)
Member, Practice Group of the Year, Competition, Law360 (2017–2021)
Member, Law Society of England and Wales (Competition Section)
Fulbright Fellow, US Department of State (2012–2013)