Credit card giant Mastercard is facing a record damages claim of up to £14billion for allegedly ripping-off millions of shoppers - and it could mean a windfall for millions of people in the UK.

Lawyers have filed a case at the Competition Appeal Tribunal against the payments company.

They believe up to 46 million Brits could be due money back because MasterCard charged shops too much for purchases between 1992 and 2008, a claim that the company denies.

The original claim in July worked out at £450 each, but when lawyers filed the case the amount they were asking for had fallen - it now equates to roughly £330 per person.

What's happening and who could be in line for a payout? We take a look:

Who gets the cash if the case succeeds?

You do (as long as you're over 18). Yes. Really. It doesn't matter whether you used a MasterCard, credit card, or anything else.

The case is on behalf of everyone in Britain - not just MasterCard customers - who bought anything between 1992 and 2008.

How you get the money hasn't been worked out yet - as this is the first case of its kind brought.

What MasterCard did

In a nutshell, MasterCard charges shops and businesses “interchange fees” when people buy products from them using its cards – they're the reason many places have “minimum card spends”.

Card providers use these fees to fund perks like cashback card deals, Air Miles and more to attract customers, among other things.

But 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 .

Previously MasterCard fees – as well as other card provider fees - were far higher than this, and that's what led to the lawsuit.

The case against them

The new lawsuit claims MasterCard set unlawfully high interchange fees for 16 years.

These were then passed on to consumers in the form of inflated prices for good and services.

Merricks argues the total damage caused to UK consumers could be as much as £14 billion, equating to hundreds of pounds for each shopper.

"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,” he said when the law suit was announced.

If they're successful that would mean money back for everyone, regardless of whether they had a MasterCard or not.

MasterCard's defence

MasterCard firmly disagrees with this assessment of the situation.

"Now that the claim has been filed, we will take time to review it in detail, however we continue to firmly disagree with the basis of this claim and we intend to oppose it vigorously," MasterCard said.

"We deliver real value through the benefits of security, convenience and consumer protection, and we are committed to investing in our payment services in order to continue to meet the rapidly evolving needs of all our customers."

When the case was first announced, Tim Murphy and Mark Barnett from MasterCard explained in more details in a statement about the case on their site.

"Our cardholders benefit from some of the richest rewards programmes, seeing real added value beyond the purchase.

"And, technology like contactless payments and our work with transport networks are driving easier payments in everyday travel.

"Merchants also receive enormous benefits from our technology, including greater security and the most significant benefit of all – increased sales.

"Of course, there are costs to making a payment. In our system, the costs of making a payment are shared by all of those who benefit from the payment – the consumer, the card issuer, the retailer and the acquirer (what the industry calls the retailer’s or merchant’s bank) – all pay a small portion of the cost.

"By sharing the costs among ALL of those who benefit from the transaction, consumers and merchants obtain the full value of our technology at costs that are much lower than they could achieve if they paid the costs on their own.

"It is unfortunate to see the US class action system being replicated in the UK. All too often in class action cases, the only winners are the lawyers. If past experience is any indication, even the lawyers will be losers in this one."

Why now?

US-based law firm Quinn Emanuel has been instructed to file the case under the new Consumer Rights Act 2015.

That matters, as this piece of legislation means people can now pursue collective damages – class action lawsuits – in the UK.

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."

What next?

"The UK has never really seen class-action style suits like this, so we are very much in uncharted territory with this,” Matt Rigby from customer complaint service Resolver.co.uk told Mirror Money.

When filed, the case is expected to be heard by the Competition Appeal Tribunal. A spokesman said the case, which is unaffected by the UK’s decision to leave the EU, should be decided by 2018.

Beyond Mastercard

The good news is that this is just the first big class action suit brought in the UK under the new rules.

MasterCard was far from alone in charging interchange fees to shops - which means if this proves successful then many more cases could follow.

With a bit of luck, we could now start to see a string of companies called to account for charges, fees and more that people view as unfair - with benefits coming to everyone who's held them.