MasterCard, the world’s second largest credit and debit card issuer, is being sued for $18.6 billion in the United Kingdom.
Court documents filed in London on Thursday allege the firm charged unlawfully excessive fees on millions of transactions when customers paid for items in stores between 1992 and 2008, resulting in higher prices for consumers.
The lawsuit was filed at the Competition Appeal Tribunal by law firm Quinn Emanuel on behalf of Walter Merricks, a former U.K. financial services ombudsman.
The law firm said the claim is the biggest in British legal history and that about 46 million MasterCard users could get payouts.
In 2014, the EU’s antitrust regulator found that MasterCard’s fees to process payments were excessive.
In a statement on Quinn Emanuel’s website, Merricks said: “MasterCard charged billions of pounds of unlawfully high fees for its sole benefit and to the detriment of consumers. There is no basis upon which MasterCard can contend that its card fees were not unlawful.”
MasterCard said it will oppose the suit.
“We continue to firmly disagree with the basis of this claim and we intend to oppose it vigorously,” it said in a statement.