In the case of Mastercard v Merricks, the UK Supreme Court confirmed the Certification Test for UK class actions brought in respect of breaches of competition law. EXECUTIVE SUMMARY On 11 December 2020, the UK Supreme Court handed down its highly anticipated judgment in the case of Mastercard v Merricks. The case concerned the certification procedure for US-style ‘opt-out’ collective (class action) proceedings before the UK Competition Appeal Tribunal (the “ CAT”). Opt-out actions automatically treat anyone who falls within the scope of the proposed class definition as being a member of the class unless they explicitly opt-out or withdraw. Opt-in actions, as the name suggests, require potential claimants to expressly sign up to be a member of the class.