British customers of MasterCard are going after the financial services corporation over charging illegally high fees on store transactions for more than a decade.

The class action lawsuit, filed last week at London’s Competition Appeal Tribunal by law firm Quinn Emanuel, wanted MasterCard to pay £14 billion ($18.6 billion) to its British customers, which it claimed paid “unlawful card charges” to the company between 1992 and 2008. The law firm estimates some 46 million people in the UK were affected.

“Consumers were unaware of the level of these fees or that they were illegal,” the law firm said in a statement. “The fees were a significant cost for retailers that was then passed on through increased prices of goods and services. All UK consumers, including cash purchasers—and not just MasterCard holders—have lost money as a result.”

Quinn Emanuel is acting on behalf of Walter Merricks, former chief of UK’s Financial Ombudsman Service, the agency that tries to resolve complaints against financial services firms.