MasterCard is facing a record damages claim of up to £14billion for allegedly ripping-off millions of shoppers.
Lawyers filed a case at the Competition Appeal Tribunal against the payments giant.
They believe up to 46 million Brits could be due money back because MasterCardcharged shops too much for purchases between 1992 and 2008.
The claim – led by former financial services ombudsman Walter Merricks – is the first filed under the new Consumer Rights Act 2015.
It centres on interchange fees paid by stores on debit or credit card transactions. It is alleged these fees were passed on to shoppers through higher prices.
The European Court of Justice in 2014 backed a ruling from seven years earlier that the fees breached competition rules on cross border transactions. The EU introduced caps of 0.2% for debit cards and 0.3% for credit cards in December last year.