MasterCard is facing a claim of up to £19 billion in damages in a UK collective action over card charges that were passed on to shoppers.
The claim - led by former financial services ombudsman Walter Merricks, who has instructed US-based law firm Quinn Emanuel - is to be filed under the new Consumer Rights Act 2015, which allows for collective damages claims.
It claims MasterCard set unlawfully high interchange fees - charged to stores when shoppers swipe their debit or credit cards - for 16 years, which were passed on to consumers in the form of inflated prices for good and services.
In 2014 the European Court of Justice declared that such fees were a violation of EU antitrust rules.
On April 29 last year the European Parliament and the Council of the European Union adopted the Interchange Fee Regulation, and caps of 0.2% for debit cards and 0.3% for credit cards came into effect on December 9.
MasterCard said it "firmly disagrees" with the basis of the claim.
Mr Merricks claimed the total damage caused to UK consumers could be as much as £19 billion, equating to hundreds of pounds for each shopper.
He said: "To be clear, there is no question that MasterCard acted illegally in the way it conducted its business, a business that affects all of us. All of us overpaid to the tune of up to £19 billion during a period lasting 16 years.
"Although most of us did not know this, experts who study the retail economy knew it was happening – and so did MasterCard.
"My aim is to get the redress to which UK consumers are entitled and to ensure that MasterCard cannot hold on to the illegal profits it made.
"This case should send a signal to companies that break competition laws at the expense of UK consumers that they do so at their financial peril."
Boris Bronfentrinker, lead partner at Quinn Emanuel, said: "This is precisely the type of claim for which the new collective action regime was established. This is a landmark case where unlawful anti-competitive conduct has harmed UK consumers.
"That harm, likely to be in the hundreds of pounds, is not large enough for any individual consumer to bring their own claim. But by aggregating the claims and bringing them on a collective basis, all UK consumers who lost out will get the compensation they are owed."
MasterCard said in a statement: "Electronic payments deliver real value to people online, in-store and everywhere.
"MasterCard is committed to providing ever more convenient, safe and secure payments to all our customers, including consumers, retailers, governments and banks."
Which? director of policy and campaigns Alex Neill said: "MasterCard has been found to be imposing illegal fees on millions of consumers and businesses across Europe.
"It's now only right that the money is returned to customers that were victims of this practice. This landmark case is using the new consumer law to bring a collective claim on behalf of individuals to get back the money they are owed."