Mastercard is facing a claim of up to £14bn in a landmark UK collective action case over card charges that were passed on to shoppers.
Today, Which? is intervening in a pivotal Supreme Court hearing that will decide whether a claim on behalf of 46 million people is able to proceed to the next stage of legal action.
Here's what you need to know about the claim.
In 2016, former financial ombudsman Walter Merricks CBE launched a class action on behalf of 46 million customers against Mastercard.
It related to the European Commission's 2007 finding that the card issuer charged inflated card fees on consumer card transactions between 1992 and 2008.
It's estimated by Mr Merricks that as many as 46 million consumers collectively lost out as much as £14bn as a result of the card fees charged by Mastercard.
This means if his argument during the collective action is accepted, claimants in the UK could be entitled to around £300 in compensation.
Which? has long campaigned for the introduction of collective redress for consumers.
The Supreme Court ruling taking place on 13 and 14 May will be a crucial landmark judgment which could mean future collective actions can be brought by consumers with a clear understanding of the standard that claims of this nature need to meet in order to proceed as a collective action.
Given the importance to consumers that the new powers operate effectively, Which? is intervening to provide additional input to the court on the importance of the regime in providing effective redress to consumers.
Which? Director of Advocacy Caroline Normand said: 'Which? has long campaigned for a collective redress scheme, but with no claim under the new regime reaching a full trial, consumers have not yet had the results they need.
'This Supreme Court hearing is, therefore, a vital one for consumers and Which? has intervened to ensure the regime achieves its purpose of providing real access to justice.'