The UK Supreme Court in a split 3 to 2 decision in Mastercard v Walter Hugh Merricks CBE [2020] UKSC 51 held that the Competition Appeal Tribunal (CAT) was wrong to reject Mr Merricks’ application for a Collective Proceeding Order (CPO) which would have allowed his opt-out collective action on behalf of 46 million UK claimants to go to trial. The judgment clarifies the law and removes the uncertainty over the UK’s new collective certification procedure which has been stalled.  Cento Veljanovski, Managing Partner of Case, was expert for Mr. Merricks.