Some 46 million people in Britain could potentially benefit from a legal case brought against Mastercard demanding £14 billion in damages for allegedly charging excessive fees.
This is according to court documents filed in London this week.
The case, brought by a former chief financial services ombudsman, alleges the payments company charged unlawfully high fees to stores when shoppers swiped their debit or credit cards and these were passed on to consumers in higher prices.
Mastercard is alleged to have done this for 16 years between 1992 and 2008, in more than 600 pages of documents filed at the Competition Appeal Tribunal earlier this week.
"This was almost an invisible tax," Walter Merricks, who is bringing the case, said. "Mastercard has behaved disgracefully in this. They have not had the reasonableness to accept that what this was doing was damaging UK consumers," he added.
Mastercard said in a statement it denied any wrongdoing.
"We continue to firmly disagree with the basis of this claim and we intend to oppose it vigorously," the world's second-largest payments network said.
The lawsuit comes after the European Union's competition regulator found in 2014 Mastercard's fees to store owners to process international payments within the EU were excessive.
Law firm Quinn Emanuel said the lawsuit was the largest damages claim in British history and would be brought under a law meaning consumers would automatically be claimants unless they opt out.
Any person living in Britain who used a credit card, cash or cheques and was over 16 years old in the period covered by the lawsuit will automatically be part of the claim.
If the £14 billion claim was shared equally between the number of eligible claimants, each person could receive more than £300 each, according to a Reuters' calculation.
Merricks in a statement said the case is a watershed moment for consumer compensation in Britain.
Merricks was head of Britain's financial services ombudsmen for ten years until 2009, helping to settle disputes between consumers and financial services companies.
Britain's banks have been caught in a range of mis-selling cases in the last five years.
They have paid £24 billion in compensation for mis-selling loan payment insurance, making it Britain's costliest scandal in financial services.
Consumers no longer living in Britain, but who lived in the country between 1992 and 2008, can opt in to the collective claim against Mastercard.
Any hearing on the case is not expected until early 2018, unless MasterCard settle it out of court.