LONDON, Aug 18 (Reuters) - A London court on Wednesday approved a 10 billion pound-plus ($14 billion-plus) class action against global payments processor Mastercard (MA.N) that claimants said could entitle 46 million British adults to roughly 300 pounds each if it is successful. The Competition Appeal Tribunal (CAT) had been expected to certify Britain"s first mass consumer class action, brought by former financial ombudsman Walter Merricks, after the UK Supreme Court overruled objections to it in December. read more The decision to finally authorise the five-year case as a collective action establishes a standard for a string of other proposed class actions that have been stalled in its wake. "Mastercard has thrown everything at trying to prevent this claim going forward, but today its efforts have failed," Merricks said in a statement. "The tribunal"s ruling heralds the start of an era of consumer-focused class actions which will help to hold big business to account in areas that really matter." Mastercard said the "spurious" claim was being driven by lawyers and backed by organisations "primarily focused on making money for themselves". Merricks alleges Mastercard charged excessive "interchange" fees – the fees retailers pay credit card companies when consumers use a card to shop - between May 1992 and June 2008 and that those fees were passed on to consumers as retailers raised prices. But Merricks failed to expand the scope of the case by adding the estates of the deceased and compound interest to the claim. Mastercard said this reduced the claim"s size to around 10 billion pounds. The claimants put it at 15 billion pounds. "The decision today reduces the value of this spurious claim by more than 35%," Mastercard said in a statement. "Mastercard is confident that over the coming months a review of key facts will further significantly reduce the size and viability of the claim." ($1 = 0.7265 pounds) Reporting by Kirstin Ridley; Editing by Steve Orlofsky Our Standards: The Thomson Reuters Trust Principles.