Claire is Co-Chair of the Competition Group globally and works in Slaughter and May’s Brussels and London offices.
Claire has extensive experience of both EU and UK competition law. She has been involved in a large number of high-profile merger cases before the EU and UK competition authorities, as well as coordinating merger approvals worldwide. On the contentious side, she has been involved in many cartel and other behavioural cases, including on appeal to the General Court in Luxembourg.
Highlights include advising:
- Vodafone on its proposed merger with Three UK and its acquisition of parts of Liberty Global
- Activision on its acquisition by Microsoft
- Suez on the public takeover bid by Veolia
- Facebook on its acquisition of Kustomer
- INEOS on its acquisition of TotalEnergies’ 50% interest in three joint venture chemical production businesses
- Google in relation to the European Commission’s antitrust investigation into its AdSense for Search advertising services, including its successful appeal of the decision to the General Court
- Motorola Solutions on its appeal of the CMA’s decision in its market investigation into mobile radio network services
Claire is recognised as a Band 1 leading individual by Chambers, a Hall of Fame leading individual by Legal 500, and a Global Elite Thought Leader by Lexology Index. Claire has been named in W@Competition’s list of ‘40 in their 40s Notable Women Competition Professionals’ and The Lawyer’s Hot 100 list. She was awarded ‘Dealmaker of the Year’ by Global Competition Review in 2017 and 2022, and shortlisted in the ‘Lawyer of the Year’ category in the 2019 Global Competition Review awards.
She is a member of the Brussels Bar (A list), as well as being qualified as a solicitor (England and Wales).
Claire is fluent in French and Russian and has good German.
Directories quotes:
- "I would say she is a real standout; she is just brilliant." Chambers UK 2025
- "Claire Jeffs goes from strength to strength." Chambers UK 2025
Recent publications include:
- Introduction: A Global Perspective on Competition in the Digital Sphere Extracts from GCR’s Digital Markets Guide - Third Edition (2023). The whole publication is available here.