Britain's former financial services watchdog is suing MasterCard for 14 billion pounds ($18.6 billion), alleging the firm slapped excessive charges on millions of transactions.

Law firm Quinn Emanuel filed the suit Thursday at the Competition Appeal Tribunal on behalf of Walter Merricks, a former U.K. financial services ombudsman.

The firm says the claim is the biggest in British legal history and could bring a payout to 46 million British MasterCard users.

The suit alleges that MasterCard charged stores unlawfully high fees on transactions between 1992 and 2008, which were passed on to consumers in the form of inflated prices for goods and services.

The suit follows a 2014 ruling by the European Union's highest court that the fees violated EU antitrust rules.

MasterCard says it will "vigorously" oppose the claim.